Dec 71 UK models changed the cans to BHH 548 and BHH 549, the previously straight air inlet tubes changing to curved. Clausager states "to comply with new noise regulations, similar to those found on non-North American models from car number 258001". Note the carbs remained as HS.

For 1972 export models other than North America the can changed to BHH 546 front and BHH 547 rear, which was the same time that the twin carbs changed to the HIF type on those models. As the only part of the filter assembly that touches the carbs is the alloy base, North America also changed for the 1972 model year and HIF carbs - but to BHH 665 front and BHH 666 rear. How these differed from other export cans is not known. All these cans also changed from straight to curved inlet tubes.

For 1974 the UK changed to the non-North American export cans i.e. BHH 546 front and BHH 546 rear when they changed to HIF carbs, and stayed that way until the end of production.

It could have been to move the front intake away from the hole in the radiator surround, which would have allowed more dirt onto the filter element, but also allowed cold air to reach the carbs. Normally a good thing - cold air is denser so carries more fuel with it for a bigger bang, but under cold and damp conditions it can cause carb icing. In the past I've deliberately piped cold air directly into carbs of a daily driver, and so has a pal on his supercharged MGB, neither of us having icing problems. But maybe the factory did get a few complaints in their tens of thousands of cars and decided to make the change. Why change the rear as well? Maybe that is just more confirmation that the two are the same!

The manual choke was originally a round knob engraved with the letter 'C', changing to one with a 'fan' symbol (representing a shutter-valve) and the word 'LOCK' with an arrow. There were two types - one in hard plastic, and another with a soft rubber moulding holding a disc with the legend and picture. One can imagine this was the later type provided for safety reasons, and is one I can remember on Minis in the 70s. North America got a 'T-handle' choke knob for the 1970 model year, with the word 'CHOKE' as well as 'LOCK' and the arrow, and all (Clausager, but see below) cars got this type from the start of rubber bumpers.

I had assumed that early versions of the 'C' knob at least were 'push-pull' rather than 'twist to lock' based on my (from memory!) 64 Mini, but Dave O'Neil has sent me a scan of a Mk1 handbook which describes it as the 'twist to lock' type. There were two types - BHH526 for North America, Sweden and Germany, and BHH653 for elsewhere. BHH526 has a safety symbol in the Parts catalogue so I suspect that was a soft knob, with the other being hard.

The shutter-valve symbol is not very appropriate and neither is the word 'choke', as they really apply to fixed-jet carbs where mixture enrichment is performed by restricting the air supply, whereas SU carbs perform enrichment by adding fuel to the same air volume.

Which type of knob (hard or soft) was provided when - and even what part number they were is pretty confusing. According to the Parts Catalogue the original UK choke cable changed from BHH653 to BHH651 with HIF carbs (and the new 18V 779/780 engines) at chassis number 332033 (some) to 336650 (all). It changed again to BHH650 at chassis number 3602301 and the start of rubber bumpers, and the 77 and later catalogue lists it as BHH2064 which suppliers show as the T-handle. However Clausager says the T-handle type was provided on all cars from the start of rubber bumpers, whereas BHH650 as indicated by the catalogue is shown by suppliers as the soft 'shutter' type. Various suppliers list the two 'shutter' types interchangeably, and even the 'hard' shutter type is shown by some for the earliest UK cars - a case of "what we have is the one you want", I'm sure.

On the 'twist to lock' types wear occurs on both the cable inner and a removable component (the 'wedge' in the accompanying pictures) of the outer. So by purchasing a new cable it is possible to fit the inner and the 'wedge' from the new cable to the old outer, leaving the run of the outer in position and just undoing the nut on the back of the dash to pull the outer forward to access the wedge, rather than removing the whole thing then having to thread the new cable (inner and outer) through the most advantageous path to get smooth operation. Of course if the old outer is damaged, or is stiff in operation because it is poorly routed, then you will have to remove and replace the whole thing anyway. May be advantageous to pull out the knob, put a spot of grease on the shaft, push it back in till the grease lines up with the wedge and twist it back and fore a few times to distribute it. Then pull the knob in and out a few times wiping the shaft each time to wipe off any excess or you will get grease on the handles of your handbag.

May 2011: John Tait in Australia found his new choke cable failed to lock after just 10 starts. Opening it up he found the locking wedge is made of plastic and had deformed!

Martin Roberts's tool to aid removal of the cable from the dash.

June 2008:

September 2018:
When export models from 1971 (18v engines) and UK cars part way through the 1974 model year gained HIF carbs, they retained the 'upside down' HS choke arrangement even though the throttle cable bracket had provision for a choke cable, which was used to secure the inner.

February 2009:

The current situation
Theoretical problems
Hot start problems?
Who supplies what?
History
Fuel hose July 2015

  The current situation - August 2018: I wrote to the Renewable Transport Fuel Obligation government department asking about E10 - whether any was being sold in the UK, and whether there were any plans to sell it. The response was:

2015: From March 2013 retailers have been allowed to sell petrol containing up to 10% ethanol, replacing the previous E5. There had never been a requirement to label pumps that carried E5, and it was by no means clear whether pump labelling would be changed to indicate E10 or not. That was for 95 octane, 97 and higher octane was classified as 'petrol protection grade' which was limited to E5, but that requirement had been due to expire at the end of 2013. The Government confirmed it would continue until the end of 2016, EC proposals to extend it to 2018 were awaiting approval, and the Government as seeking the views of interested parties in this document dated October 2013. Also I had not seen any definitive statement from an authoritative source i.e. Governmental as to exactly what E5 and E10 actually meant in terms of ethanol content.

Eventually I asked the AA these two questions and had the following reply:

So the upshot is:

• E5 can contain up to 5% ethanol, and E10 anywhere from 5% to 10%.
• Only 95 octane can contain more than 5% ethanol.
• Pumps dispensing E10 must be labelled '95 E10', so potentially can be avoided.
• In answer to a written question in July 2015 the Government stated: "The ethanol content of petrol supplied in the UK is a commercial matter for fuel suppliers, subject to the Motor Fuel (Composition and Content) Regulations 1999 which set the maximum permissible ethanol content of petrol at ten per cent, known as E10. The Regulations set no minimum ethanol content.
  "E10 is not yet on sale in the UK. Petrol sold in the UK today typically contains up to five per cent ethanol, known as E5. The Motor Fuel (Composition and Content) (Amendment) Regulations 2013 ensure that E5 petrol will continue to remain available until the end of 2016, thereby providing a ‘protection grade’ of petrol for drivers of those vehicles which would be incompatible with petrol which has a higher ethanol content.
  "Should E10 be rolled out by suppliers, we will carefully assess the compatibility of the UK vehicle fleet in determining whether to extend the regulatory requirement for E5 to remain available beyond 2016.
  "This Government recognises the concerns of owners of some older vehicles that may not be compatible with E10. The Department is in regular contact with suppliers who in turn have been asked to write to us to give at least three months of notice if they were to plan the introduction of this fuel. As yet none have indicated they have any immediate plans to introduce E10.
• Proposed EC legislation would extend the availability of 'petrol protection grade' until the end of 2018, and the Government has said it will review whether UK legislation needs to be altered if the EC directive comes into force.
• Suppliers have indicated they have no plans to introduce E10, but it is a commercial decision and certain suppliers in certain areas could introduce it at any time. So if you habitually use 95, study that pump!

  February 2015: More scare tactics. Moss claims:

  Hot start problems? June 2014: Some people are beginning to experience odd hot-starting problems and are wondering if ethanol is the cause. Vee had two bouts of this recently, near the end of the tank i.e. not a recent fill (Tesco 95), and the second time was barely warm. I checked the mixture balance and it was slightly off, but no more than it had been in the past without this problem, but since correcting the mixture it hasn't reoccurred, with the same tankful, so who knows?

August 2014: Two tankfuls (one Esso, one Tesco) caused no problems despite the hottest weather of the year, but a subsequent Tesco tank did on a pretty warm day. After repeated cranking I could smell petrol so I know it wasn't fuel starvation i.e. vapour lock. It could have been flooding due to fuel expansion, but the usual treatment of flooring the throttle had no effect (although that's usually done when it is cold flooded), and it restarted after about 10 minutes with the bonnet up. The coil was very hot, I wasn't sure about the spark, and subsequent testing showed the coil boost circuit on the ballasted 6v ignition system wasn't working - and never had been with this starter. Now fixed, only time will tell.

Then a link to an American news broadcast about the problems E15 was starting to cause in nearly new cars came my way, so I asked around about how long they had been using E10 and whether it had caused any problems, see here for the responses, which summarised indicate so far that apart from possibly lay-ups over winter - and none of those in cars, there have been no problems.

  Who supplies what? September 2016: This TR-Register page contains information on ethanol in petrol including the following:

It includes a link to download a list of TOTAL E0 sites, which can also be found here, sorted by Postcode. Bear in mind that any or all of the information above could change at any time.

My own research came up with the following:

Also this Ethanil page which contains a league table compiled from users testing petrol they have purchased, using a testing kit available from Ethanil. Interestingly Shell V-Power 99 from Durham tested at 5%, whereas 95 in Hampshire and Cornwall tested at 3%, and Sainsbury's 97/98 in West Mids tested at 1%. Also interesting is that Morrison's 95 in Humberside tested at 5.25%, which if correct should have the pump labelled 'E10 85'.

  History:

For cutting-out and misfiring while under way, always look at the tach before doing anything i.e. while the momentum of the car is still spinning the engine. If the tach is jumping about or has dropped to zero while the engine is still spinning and the ignition is still on then it's an ignition LT problem.

If the tach is still registering the appropriate engine revs then try and get in a position where it is safe to turn off the ignition while the problem is apparent i.e. while the car is still rolling, and bring the car to a halt. Then, when things are quiet enough turn on the ignition while listening to the pump. If the pump makes no sound, and it won't restart, then check for a fuel delivery problem as follows: Carefully remove the delivery pipe from the front carb - being careful to catch any spurt of fuel in a cloth so it doesn't spray on the hot exhaust. If you get a spurt of fuel then the pump is working normally, so if it still won't start it's probably an HT problem, or possibly a failed condenser.

If the pump chatters away before slowing and stopping and when turning the key to crank the engine restarts, or if there was no spurt of fuel, then the problem is fuel starvation, either because the pump is intermittently cutting out or dead, or there is a blockage in the plumbing between pump and carbs. With an intermittent pump the cutting-out or misfiring is also likely to be intermittent, or you will find that sometimes when turning on the ignition there is no chattering and no start, but on another occasion there is both chattering and starting. If the pump always chatters, and it always restarts, then it is a delivery problem which should be confirmed by the delivery check here. As before if you find you are getting one click each time you turn the ignition on and one click when you turn it off again, then the points are stuck closed.

V8 Filter:

As the car is now 40 years old and on its third time round the clock I had decided to change the fuel pump hoses, as they were almost certainly original, and some people have been getting hot under the collar about ethanol and rubber. In the event they were in perfect condition, but that's another story.

As part of that job I had occasion to remove the filter hose from the carb, and noticed it had some internal longitudinal splitting at the cut end. I cut an inch or so off and opened it out but the splitting didn't extend under where the clamp had been, so I reckoned the rest was OK. However what I did notice was where the end of the carb port had been positioned in the hose there was some radial damage, and I wondered whether it had been caused by the unsupported filter flapping around for 20 years and nigh-on 100k. That started me looking at filter clips.

A 2BA screw and lock washer secures it to the bulkhead, then the supply pipe has to be repositioned towards the bulkhead and the heater a bit to sit under the clip with the open end vertical, as it had been pointing forwards quite a bit for how the filter was previously fitted. I bought a length of 1/4" hose for between filter and carb, partly because the filter is now sitting further back so that needs to be longer, but also as a replacement anyway given the existing condition, and I still had some 5/16" left over from the pump hose replacement to fit between the supply pipe and filter. I cut the 5/16" with a small hacksaw as that was before the filter which would catch any bits, but the smaller 1/4" I was able to cut with a pair of tin snips and hopefully leave no bits.

Now for a leak check. Turned on the ignition, got the initial rapid clicking as expected while it recharged the filter and hoses, then a steady click - click - click every couple of seconds or so - not so good. No visible leaks, so perhaps a carb float valve has got some debris in it. Even though I was careful to cut the hose that side of the filter with blades rather than saw, maybe there was something inside the new hose. Took the overflow hoses off and sure enough it was pulsing out of the near-side carb i.e. the first one to get fuel. So instigate ploy No.1 which is to remove the fuel pump fuse and run the engine until it empties the carbs and stops, then reconnect the fuse. Much chattering as it refilled the now empty carbs, hopefully flushing out any debris from the now wide-open float valves. The chattering slows and stops, then silence - no more clicking. It's perhaps something I shall have to keep an ear on for a while, turning on the ignition and listening to the pump before starting the engine, in case any more remains to cause a problem subsequently.

Moss Europe have Gates Barricade 'ethanol-proof' in 1/4" dia and 5/16" dia which are the two sizes needed on the MGB. If this are an easy fit to the ports you should be OK, but if it is a tight fit over ribbed ports then the inner layer is easily damaged and debris will block your carbs. The strange thing is that these 5-layer hoses are cheaper than the standard 2-layer!

People are getting exercised about this and the perceived effects of ethanol, as well as on running and anything else to do with the fuel system. I'll say at the outset that - so far! - I've had no problems with fuel hoses on either car. Whilst the roadster has always run on higher octanes - Shell V-Power and its predecessors by choice - there is no evidence that they are ethanol free. The V8 has always been run on supermarket 95. I have changed carb and pump hoses on the roadster, but that was only for visible appearance reasons when I refurbed the carbs and fitted a new pump, and it was many years ago. And on the V8 whilst I have also replaced the carb-end hoses for visible appearance reasons, that was also many years ago, and the pump hoses are almost certainly original.

SAE J30 seems to detail the standards for fuel and oil hose, with R6 and R9 being the specific types relevant to us. R6 is the specification for 'low pressure' i.e. carb hose, and R9 for 'high pressure' i.e. injection hose. However after that confusion reigns.

This site claims SAE J30R9 is suitable for E85, and also quotes DIN 73379 TYPE 3 E. But if you search on that you get pages that say it is for E10 or E85 or 100% ethanol or 'not for bio fuel' although to be fair this last doesn't include '3E' in the description. This site claims R9 has a nitrile core, but the FBHVC (scroll down a bit) says Acrylonitrile hoses are not recommended.

Finally if this document is genuine, Table 1 together with Appendix A appears to be saying that R9 gives the lowest permeation of 15 g/m2/day with E15 whereas R6 with the same fuel gives 600 g/m2/day. R10 seems to be an 'in tank' hose so wise to avoid for external applications I'd say. Of course, whether a vendor's hose does actually conform to R9, and whether these tests have anything to do with longevity - permeation rates being a completely different issue to rupturing - is another matter.

This Fuel System Components and Aggressive Biofuel Environments by DuPont document states that Viton and Zytel-cored hoses meet 15-year life requirements as well as meeting pressure and permeation requirements, but then as the manufacturers of those materials in that time-honoured phrase, "Well they would say that, wouldn't they". This apparently independent organisation then says that although SAE J30R9 has a thin Viton inner layer (whereas one of the above sites claims theirs is nitrile), it still deteriorates, and says that motor manufacturers are switching to Teflon or PTFE-cored hoses for their superior performance. It also says that Teflon is a DuPont trade-name, but DuPont don't mention it on their page!

Nevertheless R9 is probably going to be better than R6, but then sourcing it becomes the issue. A pal did some investigation and came to the conclusion that Gates Barricade was probably the best option, however he was unable to source any in the UK even though Gates have a UK presence of some kind. He contacted a local supplier of fuel hose off the roll, apparently able to supply R9. On arrival it turned out to be a barn in a farmyard, and the hose had no markings of any kind on it.

What about the usual MGB suppliers? They seem to have hose by the metre in imperial sizes - at a fraction of the price from Holden - and on enquiring with the MGOC they tell me they supply R6 - COHLINE type 2134, with a working pressure of 10 bar and E10 compliant. Moss also show Cohline 2134, but on ordering it I received Codan BS AU 108/2/L4/C4/R SAE 30 R6 WP 12 bar. However that was from going through the menus, if you search on 'fuel hose' you get Gates Barricade listed in both 1/4" and 5.16", and despite being 5-layer as opposed to 3-layer it's fractionally cheaper! However whereas the Codan is shown as 12 bar which is 174psi, the Gates is shown as 'only' 50psi. I doubt that will be an issue at the 3psi or so that our systems operate at.

Both suppliers also have sized lengths for each position, with banjos for the pump, but they are stainless braided as per the originals. The problem with that is given the question marks over hoses, maybe it's better to have plain rubber and be able to inspect it, rather than being original. A third pal has recently bought a V8, which although having just passed an MOT, developed a massive leak which turned out to be one of the fuel pump hoses having broken away from the pump banjo spigot. Given the number of units and metres the usual MG suppliers must get through, and the damage to their finances and reputations if they were shown to be supplying inadequate hose, I can't help feeling that R6 is going to be perfectly adequate for the MGB. And given the probably original pump hoses on the V8, I'll be replacing them, and with plain hose, rather than the braided originals.

Hose Replacement: September 2015:

I had run the tank down to not much more than a gallon to reduce siphoning problems, and had considered undoing the pipe to tank fitting joint to break the siphon. But with the high position of the pump on the RB, the join between the tank pipe and the hose is level with the top of the tank anyway, so easing the hose off the pipe let air in to break the siphon and only a few of drops came out. The pump hose to carb pipe was more of a problem as having reversed the car onto the ramps just minutes before there was still pressure in that line. But several sheets of newspaper scrunched up and held under the join as I eased the hose off caught most of that.

After that it was only a few minutes work to remove the earth wire and diaphragm-end vent tube from the pump body, then remove the metal cover in the boot, remove the 12v feed, and slacken the clamp round the large grommet and pump body, and the pump complete with hoses came out from under the car after less than an hours work. I had chosen to do it that way rather than undoing the banjo unions on the in-situ pump for two reasons - one was to avoid breaking yet another seal, and the other is that being able to grasp the pump body with one hand it was relatively easy to work the hoses of the banjo fittings with a pair of grips.

I connected up the pump but before fully mounting it turned on the ignition to test the joints. As expected the pump chattered, but then kept chattering, and eventually I switched off. Pondered a bit, I was sure I had got the hoses the right way round but checked the roadster and a spare pump to make sure, and they were. Then I wondered if, because I had refurbed this pump which originally failed on the roadster, and used it to replace the problematic Moprod pump that the car came to me with, I had got the one-way valves round the wrong way and the hoses had been the other way round ever since fitting. But checked some photos I had taken at the time, and they were correct. More pondering.

I had run the tank down, but there should be at least a gallon left, and I've never had a problem picking up with previous tank and pump changes. So I took the filler cap off and pressed the end of the filler pipe against my face round my mouth to form a seal, and blew. It pressurised, and released from the filler when I moved away. So I tried that again this time with the ignition on, chattered again, chattering rate increased slightly as I blew, but didn't stop. Switched off again and more pondering.

Suddenly it struck me. I'd driven the car onto the ramps immediately before I started, engine still cold, so the float chambers were full hence the float valves closed, and so the pump couldn't pump anything through, hence couldn't suck up any fuel! So I removed the delivery pipe from the carb (it spurted as I expected from fuel remaining in the line), directed it into a container, switched on, pump chattered momentarily, then slowed as it picked up fuel and started to pump it into my container. Phew! Whilst I was at it I decided to do a delivery check to find it delivered just over an Imperial pint in 30 secs which is correct.

One thing I had noticed (towards the end after I had pressurised the tank) was a little dampness on the end of the hose where it attached to the carb port, which wasn't something I had been aware of before, and looking at the end of the hose I could see three radial cracks. So I cut an inch or so off, which left a clean cut end, and refitted that. With that clamp tightened I disconnected the coil and did a leak check by turning on the ignition again and got three quick clicks, waited for over a minute and no more, so OK. Carefully checked the carb joint, and all four pump joints - all bone dry, so finished off fitting the pump grommet clamp and cover. All done in probably 2 1/2 hours.

May 2016:

Schematics

The fuel pump is powered from one of two unfused ignition circuits - until the 1977 model year with white wires from the ignition switch. On 1977 and later RHD models it was off the ignition relay which starts off brown/white at the relay output then changes to white where it connects to the rear harness, but on LHD models it remained from the ignition switch with white wires via an inertia cut-off switch. It is strongly recommended that the pump circuit be fused in the engine compartment - more info here.

Types:

There are any number of after-market types, many of which require mangling of the mountings and/or connections to fit. Some of these make a loud chattering all the time regardless of engine requirements, and many output excess pressure that overwhelms the float valves and causes flooding unless a fuel regulator limited to about 2psi is fitted.

Mounting:
Chrome Bumper   Rubber Bumper   An Alternative Position in Chrome-bumper Cars

  Chrome Bumper:

The pump has 12v (white) and earth/ground (black) wires coming out of the rear harness, the 12v attaching to a spade on the electrics end, and the earth to a spade by the unions. The other end of the earth terminates with the reversing light and some number-plate light earths at a number-plate bolt.

Rubber Bumper:

In the boot the 12v supply wire is connected to the pump terminal. A metal box screws to the boot front wall to protect the pump and wiring from damage from heavy objects in the boot.

An Alternative Position in Chrome-bumper Cars by Peter Mayo: Added November 2011:
At the start of the long refurbishment of my 1970 GT one of my main objectives was to replace the two ageing 6 volt batteries with a single 12 volt system, this coupled with a new lightweight high torque starter motor would ensure better first time starting and a considerable saving in weight. Removing the old batteries and cleaning the metal frames I then cut and riveted aluminium sheet turning each holder into a box thus preventing the Ingress of water and road dirt, sprayed them both with grey primer and finished with colour match and drilled a new earth point. I looked at the now vacant offside battery box and realised that I could now fulfil a long held idea of re-locating the SU fuel pump from its awkward access under and forward of the offside rear wheel and install it in its new home. First job was to jack up the car and securely support it on axle stands, got to be safe when working under a car.

NO NOISE WHATSOEVER.

Pump Vents: Added December 2009:

The port in the plastic cap has a little ball in it, which tends to act as a one-way valve. The purpose of this isn't clear, ordinarily as the solenoid operates it would tend to suck air in the body vent and push it out of the cap vent and the one-way valve will allow this. When the solenoid releases ordinarily air would flow the other way, but with the valve closed air from the diaphragm side of the solenoid will be pulled past the solenoid into the cap end. The effect of this is that as the pump repeatedly operates air is pumped through the pump body from the body vent to the cap vent. As air can apparently flow past the solenoid as it releases, presumably it can also flow the other way when it operates, so in theory only the body vent is required to allow air to flow in and out as the diaphragm itself actually moves - presumably what happened in a one-vent pump. Why they went to two isn't known. One could hypothesise that the two-vent pump moves air through the body of the pump so cooling the solenoid and coil. But as the pump is only energised for less than a second once every two or three seconds at most in normal use, it's difficult to imagine the pump getting very warm anyway. Different when you have run out of fuel of course, as the pump then chatters away like billy-oh, and for maybe half a minute or more while the float chamber empty before the engine cuts out.

There is a view that on chrome bumper cars the cap vent can be left without any tubing as air only flows out of it and the valve should prevent anything getting in. But as Abingdon saw fit to provide a second length of tubing and Tee from this into the boot it seems perverse to remove it. Even sillier is another view that says you can block off the cap vent with caulk, on the basis that the early pumps didn't have it!

My pointless Moprod (same system as the SU but different packaging) has no identifiable ports at all, so either it vents differently, or maybe they didn't think compressing of the air inside the pump was an issue. It's true my pump started short-stroking, which was why I replaced it, but only after many tens of thousands of miles including very cold and very hot weather, so I don't think that was the cause.

Testing, Diagnosis and Adjustment: Added December 2009:

Electrical tests November 2014
Adjusting the diaphragm and points June 2014

If you have an SU pump then listening for clicking when turning on the ignition but before starting the engine is a perfectly valid test. Depending on how hot the engine was when last switched off it may click just a couple of times (cold) or for several seconds (hot). But if you turn the engine on within a few minutes of turning it off it may not click at all especially when cold. Note that some after-market types chatter all the time, regardless of engine demand.

If it doesn't click at all after having left the ignition off for several hours or overnight then the pump or its electrical supply is probably faulty. If it clicks, then it should stop, and only make a single click once every 30 secs or longer. If it clicks more frequently than that then either the float valves are leaking and it will eventually overflow, which if you have a charcoal canister make take some time to appear on the ground, or the non-return valve in the pump inlet is leaking. If it continues to click rapidly then either you are out of fuel i.e. the fuel level is below the pickup strainer, the pickup pipe is perforated above the fuel line, or a float valve or the non-return valve mentioned above are stuck open. If rapid clicking stops and starts while the ignition is on but the engine isn't running that implies either very marginal fuel level or the non-return valve intermittently sticking. Note that some after-market types output excess pressure which will overwhelm the float valves and cause flooding unless a fuel regulator limited to 2psi is also installed.

If it clicks and stops as it should, then check delivery. Remove a fuel feed pipe from a carb (be aware it will spurt if the ignition has been on recently with SU and Moprod types), direct it into a container, and turn on the ignition. It should deliver at least one Imperial pint per minute, in practice closer to two, in a steady stream of pulses with minimal bubbles. Erratic pumping indicates pump or fuel level problems, lots of bubbles a leak on the tank side of the pump plumbing. Note that the delivery requirements apply to after-market types as well.

If all that is right then the only other thing running the engine is going to tell you is if there is a very intermittent problem with the pump or its electrical supply that only being operated for a long time may reveal.

  Electrical Tests: November 2014 SU Burlen state that these pumps take 1.5 amps at the minimum voltage of 9.5, which implies a solenoid resistance of 6.3 ohms. However when I measure the resistance between the two pump spades of mine I see about 2 ohms, which implies a current of 6 amps at 12v, and 7 amps at a running voltage of 14v. I then measured the current through the solenoid i.e. bypassing the points and at 12v saw 5.5 amps - so where do Burlen get their 1.5 amps from? If you power a pump on the bench, not pumping fuel i.e. chattering away, using either an analogue meter or a digital meter that will average the current, that's when you will see about 1.5 amps. In other words the current that Burlen quote is the average of the current flowing while the points are opening and closing, and they are open for longer than they are closed.

  Adjusting the diaphragm and points: June 2014 The Leyland Workshop Manual and Haynes info on points adjustment is pretty sketchy, more info here. But the info on diaphragm information is confusing at best if not downright incorrect, correct info is here. Note that there have been three different methods of spacing the diaphragm over the years, and SU Burlen have gone back to the original brass spacers after two different types of plastic spacer.

Update January 2010: This source details a problem with a nearly new Burlen pump. It seems to have been caused by a misalignment between the moving and fixed contacts such that only one contact pip of the two each side was being used. On the face of it one should work as well as two (in fact much earlier SU pumps did only have one pip each side) except that physical erosion from the rubbing action as the contacts are closed and opened would occur at double the rate. I don't know how good MOV spark suppression is compared to diode (there seems to be no sparking with diode like there is with capacitor quenching) but if there is some sparking, and hence burning, maybe that is enough to cause a single contact to fail fairly rapidly. With two even if one burns and so no longer conducts, it will still be subject to the rubbing action while the other contact pair is still working, which would tend to wear any such burning off again.

Union Leaks:
Banjo union sealing changed from fibre washers to O-rings at some point, and it is important to get the correct seal for your pump body or you will get leaks. Looking carefully at the ports on the pump, the type that need a washer have the thread starting right from the face. But the type that need an O-ring have a recess before the thread starts.

The banjo has an orientation - the flat face goes towards the pump, regardless of whether a washer or an O-ring is fitted, and a washer fits into the recess that goes towards the bolt.

See also the following information from:

• Dave Dubois
• An alternative approach from Herb Adler

  Pump refurbs: In June 2014 I replaced the points and diaphragm and some other parts on a spare pump which seems to have been successful.

Then in early 2015 Michael Beswick refurbed one of his, but went further and dismantled the air bottle and smoothing chambers replacing the seals and diaphragm, and had considerable problems, as recounted here. He ended up having the body replaced, and from his experiences and comments from Dave Dubois it does seem that current bodies are slightly different in a number of ways and some replacement parts do not fit old bodies very well.

Removing the emissions plumbing (Added August 2010) Many people remove the emissions stuff, but apart from the air pump and gulp valve there is little to be gained by removing the charcoal canister other than space in the engine compartment. If you do, there are a number of things you should do as part of the job:

• Crankcase ventilation - this comes via the charcoal canister to the port on the rocker cover, and must be retained. You could seal the restricted port on the rocker cover and fit an earlier breathing oil filler cap, but that wouldn't be obvious and a non-breathing one could be fitted subsequently which would kill the ventilation. Much better to fit a small filter to the rocker cover port, then it is obvious.
• Tank ventilation - again this must not be sealed, unless a ventilated petrol filler cap is fitted, or you will get fuel starvation. You could just leave the hose open in the engine compartment, but that would emit fumes as the tank was filled and the fuel expanded. Neither should you simply disconnect the breather pipes in the boot or that will fill with fumes. Probably the best solution is to fit a loop of hose to the tank breather port, going up into the space by the battery and back down again to about axle level.
• Carb float chamber ventilation - this must be left open to atmosphere otherwise the float valves will never rise to stop the pump, and fuel will be pumped straight into the inlet manifold and engine. Neither must the hoses simply be removed and the ports left open, as any fuel venting if (when?) the float valve fails can be poured straight onto a hot exhaust. Instead pipes or hoses must be fitted to take any leakage down past the exhaust to by the bottom of the crossmember by the front left-hand engine mount.
• Inlet manifold ports - none of these must be left open to atmosphere or it will cause a significant vacuum leak. If you are not happy plugging the port itself (which must be done in such a way that the 'plug' can't be sucked into the engine) then replace it with a blanking plug and sealing washer.

Briefly 'octane' is a number which indicates the grade or combustibility of petrol. As well as being ignited by a spark e.g. from a spark plug in an engine (which initiates a rapid but steady burn), fuel can also ignite if it is compressed enough which heats it up (which causes a sudden explosion). The octane number indicates how much a particular grade of fuel will resist this compressive explosion - the lower the number the more likely, the higher the number the less likely. A sudden explosion is very bad in a petrol combustion engine, it can cause serious damage to the engine (by contrast a Diesel engine is designed to cope with the sudden explosion which happens when fuel is injected into the hot, compressed air in the cylinder). High compression petrol engines need a high octane fuel, but lower compression engines can run on a lower octane. Higher octane costs more to produce than lower octane and consequently is more expensive at the pumps, but generally delivers more performance when used in an engine that is designed to take advantage of it. Using high octane in an engine designed for low octane gives no performance benefits. However using a low octane fuel in a high compression engine will cause pre-ignition or detonation under load (similar to the sudden explosion referred to above) known as 'pinking' or 'pinging' from the sound it makes, which can damage the engine.

There are various way of measuring and specifying octane: The UK uses RON (Research Octane Number), there is also MON (Motor Octane Number) measured in a different way, and America uses a composite of these two (RON+MON)/2 which is known as PON (Pump Octane Number). Taking just one example 99 RON is 90.75 MON and 94.9 PON.

The MGB was designed to run on '4-star leaded' in the UK which equated at the time to 98 or 99 RON. Now leaded is no longer commonly available (for health reasons, although it still is from some UK specialist garages) unleaded at 95 RON is the universally available fuel in the UK with many (but not all) stations also stocking Super Unleaded at 97 RON. There is also Shell V-Power at 99 RON (replacing Optimax at 98 RON), and Greenergy from some Tesco outlets also at 99 RON. BP Ultimate 102 (102 RON) is available from some outlets but at about 2 to 3 times the price of 95 or 97 octane! Whilst the MGB will run on 95 RON the ignition typically has to be retarded by 3 degrees or so to control pinking, but this reduces the performance of the engine, increases running temperatures, and increases fuel consumption. Some eras of MGB engine seem to be very prone to pinking (my 73 is and I have noticed it on others of a similar vintage), probably due to features of the cylinder head design. Even when 4-star was still available I had problems with pinking if the timing was advanced even slightly from specification. By choice I have always used Optimax when available, have just started using V-Power, but can't say that the change from 98 to 99 has made it any less likely to 'pink'.

Further reading:

On most (if not all) models the fuel pump wire is in the rear harness that that runs under the floor and connects to the main harness in a mass of connectors by the fusebox on the RH inner wing. It is a simple matter to pull the white wire from the rear harness out of the bullet connector in the main harness, make up an in-line fuse with two short wires terminating in bullets, plug one wire into the existing bullet connector in the main harness, and with a new single bullet connector connect up to the pump wire in the rear harness.

Note that for North American cars with the gearlever manual switch the inertia switch feeds both the fuel pump and the overdrive. There is a double connector, possibly by the junction of the firewall and RH inner wing as before, where there is a white from the inertia switch, a white to the fuel pump, and a white/brown to the overdrive switch. To just fuse the fuel pump insert it in the white wire going to the pump. But while you are at it you could fuse both the fuel pump and the overdrive it you insert it in the white from the inertia switch. If the inertia switch has spade connectors you could insert it here (using male and female spades on the in-line fuse instead of bullets). Incidentally, if adding an inertia switch to car that didn't originally have one, this is the place to insert it - in series with the fuse.

Personally I would use a standard 17amp rated/35 amp blow fuse in the circuit simply because there are (or should be!) a couple of spares of that rating in the main fusebox. That rating may seem higher than required for the pump but the purpose of the fuse (like all in the MGB bar certain North American models) is to protect the wiring and that rating is fine. Information from SU Burlen is misleading when they say the pump takes 1.5 amps at 9.5v, so you may be tempted to fit a 2 amp fuse. The pump actually takes pulses of 7 amps at 14v, but only very briefly, it is only over time that these average out to about 1.5 to 2 amps that you would see on an analogue meter. If you fit a fuse of less than 7 amps then it is repeatedly being stressed, and may eventually fail when there is no fault.

This arrangement had an unexpected benefit out in my V8 one day when I had a major fuel overflow from one carb, and I really couldn't countenance driving home with fuel pouring out of the overflow. Then I had the idea of cross-connecting the overdrive and pump fuses so I could use the OD switch to turn the pump on and off! While driving along I'd turn it on for two or three seconds then turn it off again, then continue driving until I felt it start to splutter from fuel starvation, then turn the pump on again for a few seconds and so on.

  My attempts to control Running-on
The Problem or to save time you can skip straight to
The Solution
An alternative solution
Yet another alternative

  The Problem

The Solution

Several months and thousands of miles down the road there may have been a slight cough on one or two occasions, but other than that it has stopped as clean as a whistle under many different conditions.

  An alternative solution August 2010

  Yet another alternative November 2010

Non-Dieseling (normal running) runon in North American cars

The North American anti-runon valve

Valve Schematic

From 1973-on North American spec cars had an anti-runon valve factory-fitted. The ignition switch has a special contact that is closed when the ignition is off and open when the ignition is on. This contact has a slate (grey) wire feeding 12v to an in-line fuse. The other side of the fuse has a slate/pink wire going to one side of the valve, and the other side of the valve has a slate/yellow wire going to an oil pressure switch which supplies the earth for the valve. Note this is a special switch with a normally open contact that closes with oil pressure, it is not the more common type used to warn of low oil pressure that has a normally closed contact to light an oil pressure warning lamp. If you install the wrong switch the engine probably won't even start. With the engine running and the ignition on there is no 12v supply to the valve but there is oil pressure so there is an earth going to the valve. As the ignition is switched off 12v is sent to the valve, and as the engine is still running and there is still oil pressure, the oil pressure switch is still supplying an earth to the valve. So the valve operates and cuts off the supply of fuel by applying manifold vacuum to the vent ports of the carb(s) which sucks the fuel out of the jets instantly stopping combustion. When the engine stops and the oil pressure dies away the oil-pressure switch opens, removing the earth from the valve which releases. You may hear a click a couple of seconds after switch-off. So when the car is being driven the valve has an earth but not 12v, and when the car is parked it has 12v but not an earth. When you first start cranking it has neither 12v nor an earth, only during switch-off does it have both 12v and earth. But the valve will only do its job if the emissions plumbing is complete and has no blockages or leaks (see above). The valve can be tested as follows: With the ignition switch off, remove the wire from the oil pressure switch and briefly tap it on and off a good earth. You should hear the valve click as it operates and releases. If it does not, check the in-line fuse, 12v supply from the ignition switch, and continuity of the wiring and valve. (Corrected 25th August 2003 thanks Patrick Callan of Connecticut.) If the valve is clicking OK, then either there is an internal mechanical defect, or there is a leak or blockage in one of the emissions hoses or charcoal canister. If you remove the emissions kit (although there is little benefit in removing anything other than the air-pump and gulp valve, except freeing up some space in the engine compartment) and encounter this running-on problem, and have an ignition relay, then you will need to correct a wiring design error as described here.

Theory
Ports
Flange Gaskets
V8 - which carb feeds which cylinder?
Floats and Valves
Dampers
Piston Balls March 2013
Return Springs and linkages
Throttle Pedal and Cable
Choke Control
Hoses July 2015
Jet Height
Vacuum Port June 2017
Setting-up
Heat Shield March 2018
Reconditioning
Plug Colour

Done properly, and not fiddled with afterwards, SUs will keep their tune for many thousands of miles. A superb quote that I have seen attributed to Lawrie Alexander of British Sportscar Centre is that "90% of the problems with SUs are due to Lucas electrics" i.e. the ignition system. Before setting-up the carbs it is essential that the valve clearances, plug gaps, points gap/dwell and timing including operation of the centrifugal and vacuum advance mechanisms are correct and any defects causing erratic or rough running are fixed.

The MGB was originally provided with HS2 carbs for all markets, the rear having a vacuum advance port. In August 1971 with the then new 18V engine all export markets changed to HIF carbs - 18V584Z for North America, 18V581/582/583Y for other export markets. North American engines now had the vacuum source on the inlet manifold, other markets still from a port underneath the rear carb. UK or 'home market' cars stayed with HSs on 18V581/582/583F engines, then in November 1973 they switched to HIFs on 18V779/780F engines again with a port under the rear carb, and non-North American export models had the same engines and carbs. In December 1974 North America changed to a single Stromberg/Zenith carburettor on 18V797/798 engines until the end of production. Other export markets continued with HIFs and the same engines as the UK until the 1977 model year at which point all LHD cars were to North American spec - and only roadsters as North American GTs ceased production in December 1974. UK cars had the vacuum source moved from the rear carb to the inlet manifold in September 76 for the 77 model year. There were several additional carb spec changes during production.

Theory:

Ports: There is often confusion about which hose goes on which port of SUs. If you get the inlet and vent hoses reversed for example, the carb will flood petrol out of one of the ports and/or the jet.

Flange gaskets March 2017

The inlet manifold gaskets are straightforward but on the air-filter side care must be taken to get things installed the right way up. The air-filter flange has breather holes that maintain the underside of the piston at atmospheric pressure, so when inlet manifold vacuum is applied above the piston it can rise correctly to deliver the correct mixture. Originally the gaskets were 'handed' in that they must be the right way up or they block the breather holes. Later gaskets have two sets of holes so that they can go either way up. However 4-cylinder air-cleaner bases have the same breather and fixing holes as the earlier gasket, so even with the non-handed gasket the base must be fitted to the engine the right way up. V8 HIFs are different again in that there are no less than nine holes in the carb flange. The gaskets must be installed the right way up to leave the breather and fixing holes clear, which blocks all the other holes.

V8 - which carb feeds which cylinder? May 2015

Floats and Valves

Dampers

Ball-bearing suction chamber March 2013

Jet Height Added June 2012:

Return Springs and linkages:

There has been a lot of discussion recently as to how many return springs were fitted to the SU carbs, and in particular whether the choke has one. The Parts Catalogue up to September 1976 for car numbers 101 to 332032 lists one 'Spring-cable return' in the list of parts for the choke and two 'Spring-return' in the list of parts for the throttle i.e. three springs in total. For car number 332033 on it lists one 'Spring-choke return' and two 'Spring-throttle return' i.e. again three springs in total. The September 76 on Parts catalogue for non-North American cars lists three 'Spring-throttle return' and one 'Spring-choke return' i.e. four in total. The fourth spring on the throttle cable itself was only added for the 77 model year and on, maybe there had been complaints of sticking throttles. In all cases Part Number AEC 2075 is quoted, and they are in addition to the return springs fitted over the actual carb spindles.

For completeness both Catalogues show one 'Spring-throttle return' with the Zenith carb, the 76-on catalogue also lists one 'Spring-throttle return' with the pedal parts but the earlier Catalogue doesn't.

Mine (HSs) originally had four springs the fourth one being on the throttle cable. But this is hooked over the pin of the cable clamp and not inserted into a specific mounting point, as is the case for the other three springs. The attachment points on the heat shield may give some clue the Parts Catalogue for up to September 76 shows it with three tags sticking out with holes in (confirmed by various owners) which also goes to support three springs, but my 73 UK roadster with 48G Gold Seal engine heat shield only has one tag, and four holes on the bottom edge making five potential attachment points in all! However two of them are not holes but slots, so it is possible these have been added later with a hacksaw rather than a drill, which could reduce the original holes to three. But having said that, the slots are actually more logical places to attach the two throttle springs as they have a better alignment. The Parts Catalogue for September 76 on shows four tags, although whilst three are of the same size and look in about the right positions for springs on the linkages the fourth is larger and right off the rear so maybe for something else entirely. My throttle has always tended to be a bit jerky on small movements, lubricating the old cable and even replacing with a complete new inner and outer making no difference. I have temporarily disconnected the fourth spring and on a short drive it does seem to be smoother. The pedal return pressure doesn't seem to have been lightened to any significant degree, so hopefully there will be no increased risk of sticking. Time will tell.

However on a friends UK 78 (HIF) whilst there are holes on the choke quadrants and the throttle lever for 3 springs I can't see anywhere to hook a spring on either of the throttle cams. Neither is there a hole in the choke lever for a spring. There is only one hole either side in the flange on the heat-shield, assuming these are for the choke springs that leaves nowhere for throttle cam springs. However there are two holes and one tab with a hole in the centre, which suggests two of those could be for the throttle and choke levers as on HSs, leaving the two outer holes for throttle cam springs again as on the HSs, but as I say nowhere obvious on the cams to attach them. Confusing.

Vacuum Port: June 2017

Information from various sources indicates that all MGBs up to the 18GK engine in 1971 had HS carbs, the rear carb having a vacuum port on the upper surface of the throat between the piston body and the manifold flange.

North American 18GK engines in 1971, despite still having HS carbs according to Clausager, then seem to have the distributor connected to a port on the inlet manifold. They had AUD 465 HS carbs, but whether these carbs still had the port but it was capped, or whether there was no port, isn't known.

With the first 18V engines for the 1972 model year all export models gained HIF carbs. North-American models had different carbs to other export markets and did not have the vacuum port, other export markets may have had HIFs with the port.

UK didn't get HIFs until November 1973, and these did have the vacuum port. It's not clear when the distributor changed to the manifold port, Clausager indicates a carb change "not certain but possibly at the start of the 1977 model year", but 1975 models have been seen with a manifold connection.

With the exception of Japan all export models were to North American spec from the start of the 77 model year.

V8s always had HIFs with a vacuum port below the throat of the near-side carb.

Whether the port is above or below the throat depends on which way the butterfly pivots as it opens. If the top of the butterfly moves towards the carb, then the port will be at the top, to be presented to manifold vacuum as soon as possible, and this is how it is on HS carbs. If the bottom moves towards the carb then the port will be on the bottom, and this is how it is on HIFs. How the accelerator cable acts on the throttle spindle indicates which way the butterfly moves. On HSs the cable lifts a lever on the manifold side of the spindle, showing that the top of the butterflies will move away from the inlet manifold, and the vacuum port is at the top. But on HIFs the cable lifts the carb side of the spindle, showing the bottom of the butterflies moves away from the inlet manifold, so a vacuum port would need to be on the bottom. The V8 pulls a lever that is above the spindle away from the carbs, so again the bottom of the butterflies moves away from the inlet manifold, and hence the vacuum port is on the bottom. Clausager shows on p66 a 1975 UK engine with the vacuum pipe going to the front HIF carb, apparently to the top of the throat, but owners say theirs is underneath. Also shown is a capped port on the inlet manifold, so it may have been an owner change. The parts catalogue indicates the change from the cable pulling up the inlet manifold side, to pulling up the carb side, occurred at chassis number 332033 in the UK and 25800 elsewhere, which coincides with the change from HS to HIF in each case, indicating that all HIFs - if they had a vacuum port - would have had it underneath. Clausager also shows a 'post-76' car with the vacuum pipe going to the inlet manifold on p67, and on p80 he says the final carbs on UK cars 'possibly for the 77 model year' have much shorter necks to the bellhousings of the suction chambers, but the carbs on p66 and p67 seem to be identical.

Certainly the V8 suffers from fuel trickling down from the port into the vacuum capsule and rotting the vacuum advance diaphragm. 4-cylinder cars may not as the pipe always has to rise to go over the rocker cover. After two V8 failures of the very expensive capsule I fabricated a small separation chamber which I mounted above the carb port, so fuel trickles back to the carb.

Setting-up:

Note that in the UK MGBs first registered before 1st August 1975 the emissions test simply comprises a visual inspection for excessive smoke. Cars first registered on or after 1st August 1975 will fail if they emit more than 4.5% CO or more 1200ppm hydrocarbons. But note that if it can be shown the car is fitted with an earlier engine it only has to pass the visual test. As of May 2018 the rules have changed.

The basic requirement for good twin SU set-up is that the carbs should be matched - and that means matching springs, needles, jets, air flow and mixture. Springs, needles and jets should always be replaced in pairs. If you have modified the 'breathing' in any way - air cleaners, carbs, combustion chambers, exhaust - you may benefit from a different needle to standard. For example K&N filters may need a richer needle, see SU Needles.  While the earlier metal floats can be adjusted to give the same fuel height in the float bowl, it looks like the later plastic ones cannot easily be, except by placing washers between the needle valve and the housing if the fuel level is too high. The float height on HSs is supposed to be such that, with the float chamber lid held upside down, the float should just rest on a 1/8" to 3/16" round bar placed across the middle of the lid parallel to the hinge pin.

Solid needles on HS carbs must be installed with the shoulder near the top flush with the bottom face of the piston.

Swinging needles on HIF carbs must be installed with the bottom of the needle carrier, flush with the recess in the piston.

With the earlier solid needles in HS carbs the jet must be centred so that the piston drops smartly and freely onto the bridge with a metallic click. This test should be done with the jet screwed up to be flush with the bridge, if it drops cleanly here then it will definitely do it when the jet is lowered to its normal running position. If it binds with the jet raised the jet needs to be recentered.

Tip: A float valve can sometimes stick in the closed position, particularly if the car is not used for some time. Running the engine will empty that float chamber which will cause poor idle and running. Rapping the top of the (HS) float chamber with the handle of a screwdriver can often free the valve.

Tip: The opposite effect is dirt in the float valve that stops it closing when the float chamber is full and it overflows. Disconnect the fuel pump and run the engine until the float chambers empty and the engine stops. Reconnect the fuel pump and the resulting rush of fuel into the float chambers will usually clear the dirt away. If it happens again immediately either the float could be punctured and full of fuel so it doesn't float, or the valve could be worn. If it happens frequently change the in-line filter (if fitted) or investigate the causes of dirty fuel e.g. internally corroded fuel tank. Check the float height after replacing the float valve, or float.

Tip: Many HIFs, and possibly some HSs, have a 'poppet valve' in the butterfly which opens under conditions of high manifold vacuum i.e. the overrun. This was an emissions measure which simulates opening the throttle slightly until the manifold vacuum drops closer to its normal value at idle. This valve can stick open and cause a high idle, sometimes only during certain circumstances e.g. warming up and be OK the rest of the time. One of my V8 carbs was doing this so I soldered them shut, which needs minimal dismantling to perform. Some recommend replacing the butterfly with the solid item, which has the same effect plus removes a small obstruction from the throat of the carb, but needs much more dismantling and it can be fiddly to get the new butterfly to seat properly in the throat, which leads me onto my next tip.

Tip: A high idle that cannot be brought down to normal by use of the fast idle screws is not being caused by a vacuum leak. A vacuum leak only lets in air, whereas the engine needs fuel to run. Therefore, if the engine is still running when the idle screws are backed right off, there is some other problem causing one or both butterflies to be partially open. This could be one or more of the following:

• Maladjusted fast idle screws holding the butterfly open, see below for correct adjustment.
• No slack in the throttle cable i.e. the throttle pedal stop is causing the cable to hold the butterflies open. There should be 20 thou free play between the finger on the throttle interconnecting spindle and the choke spindle.
• Maladjusted throttle interconnecting clamps and spindle - one carb fully closed is holding the other one slightly open. Go through the full set-up sequence below.
• Butterfly poppet not seating - solder it closed or fit a plain butterfly.
• Butterfly not seating properly - check the carb throat seat is clean and reseat the butterfly.

I repeat: With both idle screws fully backed off both butterflies should be fully closed, and this is more than enough to cause the engine to stall.

The main adjustments - the detailed info relates to the HS but the principles apply equally to the HIF:

 
• Remove the air filters, slacken the throttle and choke bar clamps, two on each bar, and back off the fast idle screws that bear on the choke cams.
• Screw each jet up until it is flush with the bridge (NOT as high as it will go), then screw it down 12 flats (two turns) to give the basic start-point for the mixture.
• Start and run the engine up to temperature, adjust the idle screws to give a reasonable idle speed.
• Using a tube to listen to the hiss in each intake, or by using a balance meter such as Gunson's, independently adjust each idle speed screw so you get the same hiss or meter indication in both carbs while still retaining a reasonable idle speed.
• On each carb in turn adjust the jet height to give the correct mixture for your spec. For non-emissions HSs this is checked with the piston lifting pin which, when lifted 1/32", should cause the engine speed to momentarily increase then settle back down. If the speed stays up the mixture is too rich, if the engine speed immediately falls the mixture is too weak. Each carb should be adjusted independently so that it gives the correct, and more importantly the same, results. After adjusting each carb the other should be rechecked as they are interdependent. NB: Emissions controlled cars have various CO readings, but this method should not be used until the carbs are balanced for air flow and mixture, and then only by adjusting both carbs by the same amount in the same direction. Updated June 2012: If you have had to adjust one carb noticeably more than the other from the starting point, that indicates there is a problem on one of the carbs, probably the one you had to adjust more. See here for more info on jet height.
• Recheck the air balance again, adjusting idle screws independently as before if required.
• The throttle spindle clamps should be set such that there is a small amount of free play in the throttle cable and interconnecting spindle before the butterflies start to open. There is a lever on the throttle spindle that rests on the underside (HS, above on HIF) of the choke spindle when the throttle is closed. Insert a .012" feeler gauge between the lever and the choke spindle, lightly press down on the part of the clamp that engages with the throttle cam and tighten each nut. Check afterwards to ensure that the slight play described above exists, and also that there is about 1/32" end-float on the interconnecting spindle.
• Run the engine at 1500 rpm and check that the air balance is still correct. If it is not the throttle spindle clamp(s) will have to be readjusted. Persevere with this, it is important to get both air balance and clearances right. If you find this difficult to set your throttle spindles/bushes may be worn i.e. can be waggled up and down or from side to side.
• The choke operates in two phases - opening the throttle slightly first (fast idle), then enriching the mixture. Make sure the choke cable is routed such that it has a clear run when the choke knob is out, otherwise stiff operation can result.
• Adjust the choke interconnecting spindle clamp screws at the carbs such that both jets start to move at the same time as the choke is operated i.e. after the fast idle movement has taken place. See here for how the chrome bumper 4-cylinder choke cable attaches at the carb end, it is unusual in that it has a fixed inner and moving outer. There can be quite a lot of movement of the fast idle cams before the jets start to move, so set the choke knob at about 1/4" out, then with the cable inner trunnion clamp screw slackened pull the inner through the trunnion to turn the choke interconnecting spindle until the jets just start to move, and tighten the trunnion screw.
• Adjust the choke cable lever on the interconnecting shaft so that when the choke knob is about 2/3rds out the lever makes an angle of about 90 degrees with the cable, again to avoid stiff operation. You will probably have to go through a couple of iterations of adjustment of the choke lever and the trunnion to get both the angle of the lever and the fast-idle distance of the choke knob satisfactory.
• Independently adjust the fast idle screws such that as the choke is operated both throttle butterflies start to open at the same time, and gives the correct fast idle speed (e.g. 1000rpm when the engine is hot) when the choke knob is in its maximum 'fast idle' position i.e. just before enrichment starts.
• Adjust the idle screws by the same amount and in the same direction to obtain the required idle speed.
• The throttle pedal should reach the stop on the floor just as the butterflies reach fully open - the butterflies and cable should not act as the throttle stop. Adjust this with the cable clamp on the throttle spindle. Take up any free play in the throttle pedal with the bolt located near the pedal hinge, but not so much that it moves the throttle spindle off its stop (the underside of the choke spindle).
 
• Check the oil-level in the piston damper. The most sensible way to do this is to unscrew the damper cap lift it up, and press it down again. If you feel the resistance of the oil before the damper cap reaches the dashpot cover you have enough oil. If you try and maintain the oil level at the recommended top-up position of 1/2" above the top of the hollow rod you will be forever topping up which will wear out the damper cap threads and the damper cap will shoot up out of the dashpot cover under hard acceleration. The correct top-up oil is engine oil of whatever grade is correct for your local climatic conditions, e.g. 20W/50 for temperate climates. When you do have to top it up there is no point in filling it to above the top of the hollow rod as any oil above this point is rapidly drawn into the engine, hence the frequent topping-up if you try and maintain it at this level. However see Damper Oil Level

Herb Adler describes using a Colortune. I never got on with them on single-carb engines, so have never contemplated it for the MGB where you either have to buy two or keep swapping them between pairs of cylinders.

Important: Once the carbs are correctly set up only ever make further adjustments to both carbs by the same amount and in the same direction. Once you start adjusting the carbs independently you will have to go through the above set up to get them balanced again.

As periodical conformation of the mixture you can check the plug colour. These from the roadster all look pretty good to me, perhaps the back pair are just a smidgen richer. I decided to weaken that carb a tad, and given that it's awkward to turn the HS nuts with the air cleaners on I made a box-spanner, which worked a treat.

  July 2014

HS carbs are usually adjusted with the air cleaners off, as they restrict access to the adjusting nuts. However if you just want to make a small change to the mixture it's a bit of a palaver to take them off and replace them for something that only takes a second or two. There is a pukka SU spanner, but it's flat, albeit heavily cranked, so I can't really see it being much easier with one of those. So I decided to make a 'box' spanner to do the job, and found an offcut of some 15mm square tubing that I reckoned would suit. I have a spanner which is the correct size for the adjusting nuts, so used that to find a bolt with the same-sized head, to use as a mandrel to form a hex in one end of the square tubing. I used a steel wedge to open out the sides of the square section until I could start to get the head of the bolt in, then hammered it in, and used hammer and vice to make a hex end that was a good fit to the bolt head. The jet has a plastic part below the adjusting nut that carries the enrichment lever that pulls the jet down when the choke is pulled, so I cut one flat off the hex, and for a little way down the tubing, to clear that. Then below the five remaining sides of the hex I cut the edges back half a flat further, to give some 'swing' room to turn the nut one flat at a time. The square section at the lower end gives quite a good grip to turn the nuts - but they shouldn't be stiff anyway, and it works a treat. But if you wanted to you could fit a short tommy-bar through the lower end, or maybe even the handle of a stubby screwdriver, to give a better grip.

Heat Shield:

Originally a thin sheet-steel plate with two blocks of asbestos on the back to shield the carb float chambers from exhaust manifold radiation. Asbestos not such a good idea, and current replacements have two pieces of woven material silvered on one side. Can't say I'm impressed with the thickness, time will tell if it's adequate. Bee's has always been pretty manky and I knew almost half of the rear block was missing, so as part of the head conversion to unleaded I decided to treat her to a new one. Spring holes and tabs a bit different to the original - but at least the original ones are present, plus two more tabs in that area. One large tab near the right-hand edge not there, but then it wasn't used so doesn't matter. The new one has two off-set holes near the top edge, for what I can't imagine.

November 2018:
Even though the silvered cloth on the back of the new shield is rather thin compared to the old asbestos - I did wonder if it would be thick enough, despite some very hot weather in the summer this year I had no problems starting or running as usual. Unlike some with summer running problems who seem to think that vapourisation or vapour lock is 'common' and down to the fuel. I'd love to get my hands on one of these.

Air filters:

4-cylinder cars have a gasket between the air-filter box and the carb flange. All the ones I have seen have been handed in that they must be fitted to the right way up so as the auxiliary holes in the two flanges are clear (holes uppermost), and not blocked which apparently affect carb operation. This is frequently mentioned as a likely cause of carb problems. While doing the clutch change on a friend's 78 I found these gaskets, which have holes in both upper and lower positions, and so cannot be fitted the wrong way round. However! You still have to fit the base-plate for the air-cleaner the right way round or the holes will be blocked whichever gasket you use or how you install it! It wasn't obvious from the running of the car that they were blocked, but Keith's car failed it's emissions test this year having passed just a year earlier, done very little mileage since, no changes other than a new choke cable (which was fully releasing the choke) even during the clutch change apart from having the air-filters removed. When I went to setup the carbs for air-balance and mixture I found the filter bases upside down, hence the auxiliary ports were blocked. In the end the balance and mixture were just about spot-on, only the balance under choke was out, which wouldn't have affected the emissions test anyway, so I can only assume I put the filters on the wrong carbs hence the bases upside down, and that was enough to affect the reading. Putting the air-filters the right way round (and hence fitting the bases the right-way round) and a precautionary weakening of the mixture by just 1/8th turn to show 3.8% on my Gastester passed the retest at almost the same figure.

Originally the air-filter cans were clearly handed and as long as you kept them, the bases, bolts and gaskets together as two assemblies you are unlikely to get them on the wrong carbs. But at some point they seem to have been modified so the cans are identical, so if you keep them together but mixed up and fit them to the wrong carbs you will get the bases upside down. The Parts Catalogue shows different part numbers for the front and rear cans for all years, but you can definitely fit the later ones on the wrong carbs and it isn't immediately obvious. Originally common to all markets they changed for the 72 model year (from straight intakes to curved?) to three different sets for UK, North America, and the rest of the world (export cars changed to HIF the previous year so it wasn't to do with that). UK changed to use the 'rest of the world' set in November 73 with the 18V 779/780 engines and all twin carb engines had those to the end.

Originally ARH176 wedge-shaped with flat top, curved bottom and rounded off front and rear, with separate end-plates, 10 Imperial gallons capacity, attached to the body with longitudinal straps under the tank, and with a sender attached by six screws. In March 65 at chassis number 56743 it was replaced with NRP2 a bowl-like pressing with separate top of increased capacity of 12.7 Imperial gallons, attached to the boot floor with a series of studs and bolts and nuts around the top edge flange, and the sender attached with a locking ring. Both types had separate fuel outlet and sender ports on the right-hand side, a vented fuel filler cap (until Oct 69 for North America), and a drain plug until Jan 74. However my 73 roadster came to me with a tank without a drain plug when it was about 15 years old, and was an 'old' tank as it rusted through shortly after.

The outlet pipe exits quite high up the side of the tank and internally goes across to the middle as well as down. If this pipe corrodes through then the pump will suck air when the fuel level drops below that point, which will result in fuel starvation if not total failure to run as if the tank was empty, but you will only be able to get a few gallons in before it is visibly full at the filler. This renders the tank scrap if prior to 1977, from then on there was a combined pickup pipe and gauge sender so it can be replaced. Well, I say scrap, but you may be able to seal off the original fuel outlet on the side and fit a 77 and later combined sender and pickup. It depends on how any internal baffling on the earlier tank interferes with the disposition of the later sender.

With a non-vented fuel filler cap any expansion that does occur expels air (and petrol vapours) from the breather port, through the separation tank and into the charcoal filter which traps the fumes. These fumes are subsequently drawn off and burnt via the crankcase breather system.

From 1975 North American cars had exhaust catalysts and so were restricted to unleaded fuel, which necessitated a reduced diameter filler pipe with different filler cap to suit, and an 'UNLEADED ONLY' label adjacent, and on the fuel gauge.

In August 76 all cars had a modified tank with combined sender and fuel feed port. In late 77 California required a modified filler tube and connection, which was soon commonised on all cars.

  August 2010: A couple of issues recently.

The second issue relates to cleaning and sealing the inside of the tank, for example to stop rust particles affecting the pump and carbs and further rusting perforating the tank. Sounds like a good idea, but there have been a couple of instances where this has blocked the pickup. One chap recently then spent a lot of time disconnecting the pipe from the tank outlet to poke a wire down but he couldn't get it round the angle of the outlet connector. He then drilled through the side of the connector in an attempt to get a wire down the pipe, but it went so far and stopped without clearing the blockage. He then cut the connector off the tank in an attempt to get the pickup tube and strainer out of the tank to clear it, but failed in that as well as the strainer is retained by a strap on the bottom of the tank as can be seen above. After all that time and effort he had to shell out for a new tank, when the old one would have been perfectly serviceable for years! Even if it didn't block the pickup the sealant could block the bleed vent of the anti-overfill chamber which will cause completely filled tanks to overflow on heat expansion, but because of the sealed filler cap this expansion will go through the vent to the charcoal canister and leak onto the ground. And if that vent has been blocked by the sealant, then it will escape via the pump overwhelming the carb float valves and flooding them. Even without heat expansion with the external vent blocked there will be no route for air to replace fuel as it is used, and you will suffer fuel starvation. Note that the 77 and later tanks with the combined sender and pickup can have this removed for sealing, but you could still end up with a blocked bleed vent in the anti-overfill chamber and a blocked external vent. You could continually blow air through the external vent to keep it clear while the sealant hardens, but there is no way of keeping the bleed vent in the anti-overfill chamber clear. UK 77 and later tanks can be sealed with impunity, once the sender and pickup have been removed, although again earlier tanks might get away with blowing air through the pickup tube until the sealant has hardened.

December 2015: Another problem.
David Jackson has had problems with a new Heritage tank (strap-hung) as fitted to a 1964 car. His car uses the early Smiths stabilised gauge system with a sender screwed to the tank, which was only used for a few months. This sender float arm moves differently to the Jaeger and later locking-ring senders, and for some reason does not have the full travel that it should, so the gauge does not read correctly. The upshot is that he has had to fit the earlier sender for the undamped Jaeger gauges, but that operates on a completely different principle, more details here.