An exclamation mark as more and more people are jumping to this as a cause of running and hot-starting problems, without any diagnosis. Americans talk of 'vapor lock' and vaporisation and I think this is where people have picked it up from.
The first thing to remember is these cars have run in desert states - America, Australia and the Middle East for decades without these problems.
The second thing to remember when people start blaming ethanol is that America has had oxygenated fuels for far longer than we have, and in greater concentrations, and again don't report problems.
The third thing to remember is how our fuel systems work. We have a pump at the back of the car applying 3psi or so to the fuel. At the front we have the carbs, with float chamber and float valve, and the top of the float chamber is open to atmosphere via the vent port/over flow. No one has yet explained to me how, with 3psi of pressure from the pump, and the carbs open to atmosphere at the front, the pump can't push any air in the pipes out of the vent, and keep pushing until the float chambers fill up with fuel and the float valve closes.
Yes, modern fuels do have higher volatility, but as the vast majority of us don't get the problem it can't be the reason a few do.
Yes fuel can boil or vaporise, in hot weather I can hear it boiling at switch-off in the V8 carbs. But if that happens in the pipes why isn't the fuel pump pushing it out? The only way the carbs wouldn't fill is if the fuel was vaporising at the rate of a pint every 30 seconds, which is impossible in the relatively short length of pipe that is exposed to heat, and even if it were the pump would be chattering away like billy-oh.
Yes Americans do get vaporisation, but in cars with engine driven pumps which are pulling fuel from the back. With the pump pulling the pressure is lowered, so it is more likely to boil. With our pusher pumps the pressure increase reduces the chance of boiling. Also an engine-driven pump is only running at engine speed, so with vaporisation at idle they can get fuel starvation from the pump not being able to keep the float chambers full of fuel. Our electric pumps should be capable of delivering a pint in 30 seconds whatever the engine speed. There are any number of American YouTubes around talking about vaporisation/vapour lock and percolation (?). And the recommended solutions? Fit an electric pump at the back!
There is also the question of just how hot things have to get to cause a problem. With the best will in the world the UK doesn't get heat waves very often. But when they do, on organised runs that can get 100 or more MGs, one would expect to find several cars stuck by the roadside or failing to start if it were a generic problem, and I've never come across anyone with that problem. Very occasionally something else (I always stop and check unless they have other cars with them), but never that. Incidentally problems hot-starting are unlikely to be vaporisation anyway, more like fuel expansion with heat-soak and overflowing into the inlet manifold i.e. flooding.
Another possibility is when at switch-off heat-soak causes the fuel in the line between pump and carb to heat up and expand, which can increase the pressure to higher than normal pump pressure, and if the float chambers are full with the float valves closed that could force them open raising the fuel level there and in the jet. But there is more chance of the fuel in the float chamber expanding and raising the level in the jet, possibly to the point where it overflows into the inlet manifold causing a grossly rich mixture at restart. And it was only offered as a theory anyway, from tests sponsored by the FBHVC carried out by Manchester University on an XPAG engine. Some notable observations:
- "At a temperature of 75°C 20% to 30% of the classic petrol would have evaporated. While at 75°C nearly twice that volume of modern petrol has evaporated." But what does that actually mean in an engine?
- "With over 40% of modern petrol evaporating at typical under bonnet temperatures, it is surprising classic carburetted engines manage to run at all." But as we know they do, and most of them without problems in the hottest weather - in the UK at least.
- "Two thermocouples, one in each carburettor, at the bottom of the transfer pipe connecting the float chambers to the carburettor body (shown in photo 2 and photo 3). Typically, this was 42°C, which was surprisingly low considering that this part of the carburettor is positioned under 1" away from the 400°C exhaust manifold."
- "When the engine was running, the highest petrol temperature of 42°C was in the transfer tubes. At this temperature, less than 10% of modern petrol will evaporate, insufficient to cause any problems."
- It puts running and hot-starting problems down to two factors - "As the petrol in the carburettors gets hotter, more of it boils. The pressure of this vapour forces petrol out of the carburettor jet, which collects in the inlet manifold making the mixture temporarily richer. The vapour bubbles in the jet then cause the carburettor to deliver a much weaker mixture when the engine is running or cranking." I.e. one enrich the mixture and the other weakening.
- It suggests: "However, if the problem is not too bad, it is possible to nurse the engine back into life using the choke to enrichen (sic) the mixture." which is all a bit vague as you wouldn't know whether the problem was excessively rich and choke would make it worse, or excessively weak. But this doesn't seem to have been quantified, only theorised from the temperature readings.
- It has been suggested that adding a return pipe to MGB fuel systems will prevent problems: "Modern cars do not suffer from these problems for two reasons. Firstly, the petrol in the pipes and injectors is held under high pressure, which increases the boiling point. Secondly, as soon as you switch the ignition on, the hot petrol in the engine bay is recirculated back to the fuel tank, allowing the engine to start on a new charge of cold petrol." But the return pipe for a carb-equipped MGB would have to come directly off the feed pipe, not through the carburettors, so it cannot have any effect on what is inside the float chambers, jets or inlet manifold. Injectors are completely different as there is no reservoir and the circulating fuel is within a few mm of the nozzle injecting it into the throttle body or intake.
- Insulating fuel pipes has also been mentioned, about which they say: "Unfortunately, insulation does not stop the transfer of heat, it only slows it down. Once the engine has stopped and the petrol is no longer flowing, the petrol will heat up, no matter how well insulated the parts of the fuel system are. Benefits will only arise if the heating is delayed for a sufficient time to allow the under-bonnet temperature to fall below 45°C."
- As far as preventing vaporisation problems goes their tests on various fuels show that higher octane vaporises less quickly than standard 95 octane. I've always used higher octanes in the roadster, but only ever supermarket 95 in the V8, and only once in the V8 have I experienced anything that could possibly have been put down to heat effects on fuel, and that was flooding not vaporisation.
- An Australian writes: "Temperatures measured are no where near what we experience during an Australian summer. This makes me question how does my MG TF run when the ambient air temperature is 45 degrees Celsius. No modification to the original design specifications have been incorporated, I do use 98 octane rated fuel." To which the author replies: "Australian summers - The distillation curves shown in the article are for UK winter fuel (intended to work at around 0 degrees Celsius). I would be very surprised if fuel in Australia were not supplied in different volatility grades chosen to match ambient temperatures, just as we get winter/transition and summer fuel in the UK. Hence the reason your TF continues to run in your summer." To which I can only say "Can it really be as simple as that? And why don't we have summer grades that allow UK cars to run in hot weather?"
Over the years I've done some simple tests in hot weather:
- In the roadster at 26C ambient the engine compartment in stop-start traffic got up to 50C.
- On another occasion of 30+C ambients the V8 engine compartment on two days got up to 64C.
- Testing the V8 cooling system shut in the garage on a 30C day (exhaust piped outside), at a fast idle (from wedged throttle), I measured the air going into the grille at 41C.
The bottom line? I would love to get my hands on a car where the owner is claiming vaporisation as the cause of his problems, but until then - with the possible exception of vaporisation in the exposed jet-pipe of HS carbs, I simply don't believe that it's even possible in our cars. Even with the jet pipe it is at atmospheric pressure both ends, so apart from when actually boiling the levels in the float chamber and jet should still be the same and hence at a normal level. If it is the jet pipe that is causing the problem, then holding a wet cloth round both of them should pretty-well instantly get the car going again. I did wonder about my new heat shield as it only has a thin layer of shiny cloth as the insulator instead of the asbestos slabs, but just as there were no problems with the old one despite having a large chunk of one of the slabs missing for 30 years, there have been no problems with the new one.
An example? September 2024 As above I've said for a long time I'd love to get my hands on one. Well, I've still not done that but the next best thing is Bill in Philadelphia USA who has had hot-starting and running problems for years - getting him to try various things and come back to me with the results. Not ideal, as the way one person perceives and explain things can be quite different to another, especially when we are 'two nations separated by a common language'. He hasn't yet offered to pay for flights over there but he has said he would put me up on more than one occasion!
I've been in communication with Bill for years about various aspects of his two MGBs. One of them - a 76 converted from single Stromberg to twin HS SU - has had continual hot-start problems as well as stumbling and cutting-out at junctions and similar. The other is an earlier model also with HS SUs and does not have the problem - which makes makes him an ideal candidate to back up my contention that hot-start problems are not generic to the MGB but due to circumstances on a particular car. Every summer this problem comes round and this year was no exception, despite adding heat insulation to the float bowls, fuel lines in the engine compartment, stainless steel heat shield, and others.
He is determined to find and fix the cause, and I'm equally interested as hopefully it will help others with the same problem. But one of the difficulties in remote diagnosis is that it's only when Bill thinks to mention something that I can get a light-bulb moment. A case in point was when we were investigating the emissions control system his car has with the carb vents/overflows piped to the charcoal canister, when he mentioned that on removing the hose from the front carb there was liquid fuel in it. Never mind 'light-bulb' but a battery of flood-lights suitable for a football pitch! There should never be fuel from the vent as that indicates there is or has been an overflow, and with the jet and bridge at a lower level than the vent port fuel must be coming from there and running into the inlet manifold as well - exactly my theory of flooding and not fuel starvation. The question is what caused that overflow.
We agreed he should check the float heights, but he chose to replace them with adjustable StayUp, and new float valves. Not something I would have done as changing things can cause other problems.
The most obvious test if you suspect the float valves is to turn on the ignition but not start the engine - removing the wires from one of the coil spade terminals first to prevent the coil overheating (some electronic systems don't need that but best to be on the safe side) - and timing the clicks which once the float chamber has been topped-up shouldn't be more than once every 30 secs. (This is to check seepage through the float valves as they wear. Slower than once every 30 secs is fine as that is less than the consumption at idle so the fuel level won't rise and flood no matter how long it is left. More frequently than that and more fuel could be seeping in, which will raise the fuel level and make the mixture richer, which will affect the idle speed as well as eventually overflow. Bill did that and reported that it clicked six to eight times, then started chattering away ten to the dozen at which point he switched off (his other car has never done this". Six or eight times in 30 secs is way too much, never mind the chattering. I asked him to do it again with the overflow hoses removed from both carbs and to watch and listen, only switching off if fuel started to flood out of one of the carbs.
He did this test after changing the floats and valves, and this is the main problem with remote diagnosis - it was only because he had sent me a video this time that I realised the situation was far far worse. The six or eight click he mentioned were as soon as he switched on for a start, which would be fair enough if the car had been standing for some time. But it was every time he switched on, even when it had been turned off then immediately turned back on again (see first section of the video here). There had been suspicions about a Hardi pump not working correctly and when he sent me pictures it had been fitted incorrectly with both ports facing straight down, which traps air and causes the pump to click more frequently than it should, as had been the case with local pal Geoff. Rather than move the pump, which had been installed in a different place anyway, he decided to go the whole hog and fit a new SU in the original location and eliminate the pump as a cause altogether, with strict instructions from me as to how it needed to be orientated i.e. with the outlet port above the inlet so it can purge itself of air. Another thing I noticed in a photo was a fuel filter between tank and pump which also raised a red flag (Why? What problem was it intended to solve?). Back to the video, he started the engine a number of times and on two occasions the initial idle had a stumble and needed a rev to get it to stabilise at about 900-1000 rpm, which seems to be needed with modern fuel. Previous videos had shown the idle at barely 500-600 which I think is too low, but he hadn't altered the idle screws - more indications of mixture problems when there is the low idle.
As if that wasn't bad enough he sent me another video with the ignition left on and the pump chattering away like billy-oh, exactly as if it were pumping fuel, but with nothing coming out of the vents. It was either going onto the floor (which it wasn't), it couldn't have been pumping out of the jets at that rate with the pistons fully down and it was going for several minutes, which left only one thing - the pump itself, in the shape of the non-return valve in the inlet stuck open so the diaphragm was repeatedly sucking the same fuel from the tank and pushing it back again. Dirt in the valve? Remember that filter between tank and pump? It's still there with the new pump and a picture of the fuel level in the float chamber also showed what looked like debris at the bottom despite a fuel filter in the engine compartment as well. He is going to clean that out, then we will have to watch for it coming back. Chattering can also be caused by air in the pump or pipe to the carbs (slack input hose connections? Perforated pick-up pipe inside the tank?) when the float valves are fully closed, but that should never happen after the engine has been run without disconnecting hoses or pipes. Another day showed just a few clicks when switching on, and the 'debris' turned out to be marks in the casting and not bits so the test will have to be repeated.
The next test to conclusively prove the 'continual chattering' problem (if it happens again) to the pump and not the carbs is disconnect the fuel feed from both carbs and block it, then repeat the 'ignition on not started' test. Now there should be no fuel flow at all, only reverse seepage from the one-way valve in the pump, and there could be many minutes between clicks of the fuel pump.
Bill sent me three more videos. The first is a delivery check i.e. the carb feed to the front carb removed and directed into a container - and the result was shocking, proving that a video (to update the adage (all adages are 'old' by definition)) saves a thousand words. Turning on the ignition the pump was chattering away like billy-oh, with only fits and starts of fuel coming out, and very discoloured. It took 20 seconds before the pump settled down to a more normal rate with continual pulses of fuel, noticeably cleaner, and no obvious bubbles. This video has an example of the frequent clicking at switch on which shouldn't happen unless the ignition has not been on for a week or more (clip repeated three times) in the first section, and the delivery check in the second section:
The second video was with the feed into the container blocked off and the pump clicked about once every 10 secs - three-times more frequently than the 30 seconds minimum between clicks it is supposed to be. The third video was with the feed connected back up to the front carb, more clicks than would be normal turning on the ignition but then the hose had been off the carb. When that stopped clicks every 10 secs as before. So either the rear carb float valve was seeping in videos 2 and 3 and the front carb not contributing to it in video 3, or the float valves are OK and it is the one-way valve in the pump that is seeping back. The first question is why the fuel was so discoloured at the start of video 1 - as it ran clear shortly after then that can't be coming from the tank, so it must be occurring as it is sitting in the lines and the pump when parked up. Pipes? Hoses? The second question is why it is taking 20 secs to start delivering fuel properly - chattering away like this indicates there is no fuel in the pump and it was having trouble pulling it from the tank. Air leak on the inlet side? Pump banjo/bolt incompatibility?
At another switch-on to take the car for a good hard run the pump chattered away for 20 secs and showed no sign of stopping so he started the engine anyway. Just after that the chattering slowed from very rapid (as at the start of the delivery check in the above video) when no fuel was coming out, to regular pulsing (when it was in full flow), then stopped. I've had that myself after having disconnected and reconnected things at the pump. So I'm convinced that, somehow, his pump is emptying while parked i.e. air is getting in from somewhere. On his return from the 'good hard run' there was again liquid fuel in the carb vent hoses, so he still has the overflowing/flooding problem as well.
We keep going, the 'chattering' symptoms i.e. lack of fuel reaching the carbs would seem to be opposite to those needed for hot-start problems i.e. flooding, but he still has the latter as well. You never know, when we find and fix the chattering issue the flooding issue may get worse and be easier to find! Hope springs eternal (but hopefully not pushing fuel into the inlet manifold ...).
October 2024: In an effort to get to the bottom of Bill seeing liquid fuel in his overflow hoses I've more than once asked him to do an 'ignition on engine not started' check with the hoses off the carb overflow ports. Since from the above video it became obvious how useful a video can be in remote diagnosis I've also asked him to film the test. The test needs the wiring to be removed from one of the coil spades to prevent that over-heating with points and some electronic ignition systems (but best to remove it on all cars to be sure). The object of the exercise is to time the clicks from the fuel pump, and watch the ports for any fuel appearing. The clicks should not be any more frequent that once every 30 secs (after an initial few depending on how long the ignition has been off) in any event as that does mean that either fuel is seeping past the float valves (or the non-return valve in the pump inlet is leaking back). Even where the clicks are more than 30 secs apart it could mean that feel is seeping past the float valves, but in normal use i.e. engine running even at idle it should still be using more than that i.e. it will never overflow. However for whatever reason that hasn't happened so far. Instead Bill replaced both floats and valves in both carsbs but still had the same issue and wondered if it could be a faulty float chamber lid which of course it could be and was already in my mind. He ordered a 2nd-hand lid for the front carb, but had also been asking the chap he bought the floats and valves from what could be wrong and sent his lids off to him to have a look at. He said both had damaged tips and he had replaced them and was returning the lids. However he has had a major incident at home so at the moment anything and everything to do with the cars is on hold.