September 2024: The car in question is a North American spec 76 converted from single Stromberg to twin HS SU. Some of the problems could be attributed to vaporisation which many say is the cause of hot-starting issues but he has another, earlier, model also with HS SUs and that does not have the problem. This makes makes him an ideal candidate to back my contention that hot-start problems are not generic to the MGB but due to the specifics of 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. Note that while insulation can delay how quickly things heat up ultimately they will reach the same temperature as uninsulated.
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 as the cause of hot-starting problems. 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. 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 than I had believed. 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 on, engine not running, and the pump was clicking away 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 carbs 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.
As I mention at the outset he has two MGBs with twin HSs and the other one hasn't had any problems of this nature, which surely indicates very strongly if not proves that his issues at least are down to this specific car and are not generic. However he has had a major fire at home where the earlier car was destroyed and the 76 damaged which brought any further testing and diagnosis to an end, with no resolution.
January 2026: After a long delay due to the fire we have picked up the baton again. He sent the StayUp floats and lids to a specialist who weighed the floats and found one was 7.8gm and the other was 14.5gm - obviously something wrong somewhere. Testing various StayUp and standard floats the average came out at about 7gm for both. So two new floats were fitted by a local workshop and various testing indicated that problem had been solved.
April/May 2026: Except it hadn't. After the carb floats were found with vastly different weights they were replaced - but no change. A British car mechanic found the pump pressure at 4psi which is on the high side so fitted a pressure regulator set to 3psi. Early indications are that overflow is still occurring but on just one carb now, so there is still some issue with floats and valves as 3psi should be fine but we are also looking at possible issues with the North American canister system. He has early HS4s used before the canister system (on what was originally a Zenith-equipped car) and the British mechanic was of the opinion that you can't mix and match and he should vent/overflow down the side of the engine as per earlier practice. I can't see the difference as both systems should be venting to atmosphere albeit the early system directly through pipes hanging down and the later via the canister (and anti-runon valve in this case). One theory has it that the PCV system crankcase vacuum continually drawing fresh air through the canister to purge it is sucking fuel out of the float chambers via the overflow. But the rocker cover has a restriction hence a very small air flow and everything beyond that i.e. canister, tank and float chambers should be at atmospheric pressure - and in any case if that's the cause "why don't they all do it?".
However there could be something wrong with his system somewhere i.e. a blockage or excess vacuum so he is currently trying plain hoses down the side of the engine ... and the first results from that is one of the hoses still has fuel in it after a run! which points the finger back at pump pressure, float and valve. And then from a new video it becomes immediately apparent that there is a bad misfire when sharply opening the throttle - regular beat so possibly one carb not delivering fuel and yet another issue:
I suggested removing the air cleaners and watching both pistons as the throttle was opened smartly to see if they moved differed, as well as with engine off and at idle.
However he decided to take it to the British car mechanic who reset the floats to 1/8" as he considered they were too high. Bill had sent me this picture of one of his float lids, and to me the distance between the float arm and the lid is at best parallel and possibly increasing as it goes from pivot to float whereas a drawing from SU shows it decreasing. Now photos are difficult to interpret and drawings are not always dimensionally accurate but to me a float with an increasing distance will result in a lower float and hence fuel level than one with a decreasing distance i.e. potentially the SU float will give a higher fuel level than Bill's.
Blipping the throttle one carb piston wasn't moving due to a badly centred jet so the needle was binding. Recentering solved that but showed rub marks on the needle so strictly speaking that and the jet should be replaced - and probably both to keep them the same. But best to run with it for a while to avoid changing too many things at once.
All plugs showed very weak, jet depths checked and found to be quite a bit out so reset to 0.085" (or 0.090", he's not sure which) but information on what they were to start with is not clear. Reset the mixtures and air-flow balance. Valve clearances 'didn't sound right' so they were checked, one found 'very large' and reset as well.
It was immediately apparent that pickup was miles better with no hesitation as previously. Bill wanted to put the canister hoses back on the overflows but I strongly advised he stay with the plain hoses for the time being so he could check for overflows more easily. I wanted him to put small containers under each and check them before and after each run but not done yet.
The mechanic also advised that the mushroom PCV be moved from the front port on the inlet manifold to the middle port, where the servo hose was attached, so the crankcase suction was balanced between the carbs. I agreed solely on the basis "that's how they were originally" but feel it would make very little difference if any to running.
He also recommended that the non-vented oil filler cap be replaced with a vented saying "Because the only air entering is through a VERY small hole in the valve cover nipple to canister hose. A vented cap will allow more air if/when needed." Yes it would, but more air should never be needed, and by increasing the air flow into the inlet manifold it would act like a vacuum leak and upset the mixture balance across the throttle range. He also said "Again, all MG engines that used PCV had vented fill caps" which is incorrect. Only non-North America spec engines with PCV had a vented filler cap all through production (before PCV all filler caps were non-vented). As soon as North American spec got the charcoal canister they changed to the non-vented cap to ensure the crankcase drew air through the canister and purged its charcoal filter of fuel fumes, and stayed with that until the end of production. More on crankcase ventilation here.
Finally the servo hose is actually heater hose with the ends cracking a bit. Not only could heater hose collapse under the vacuum but cracked ends could be letting in air and creating a vacuum leak. The mechanic recommended replacement using "power steering hose or similar", but probably best to get pukka servo hose from one of the usual suspects, which may need the carb mixture resetting.
So a number of problems found and hopefully fixed and now it's a case of doing some decent running and see how it goes. Even if the hot-starting, stalling and overflow issues have been resolved I can't see the work that has been done resolving the issue of pump clicking but delivering no fuel.