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MG MGB Technical - Stroppy Engine

My 1972 B with rebuilt engine will start OK from cold and drive perfectly for however long I want. But if I then stop, for say 5 to 10 minutes, on restarting the engine runs very roughly with poor, stuttering acceleration and can just about get me home. It will then refuse to start until the engine has completely cooled when it will act as though there is nothing wrong. Crypton tuning shows no problem. It runs on unleaded fuel.
C K M Edwards

Almost certainly a vapour lock - next time it does this, take off the float chamber tops (if HS4) and check the fuel level.

Unleaded is prone to this.
Chris at Octarine Services

CKM,
Is the heat shield in place between the carbs and manifold. Without it the carbs will overheat and vapor lock will occur as soon as you stop driving.
Bill
Bill Boorse

Could be heat soak into the dizzy causing the capacitor to fail . Usually with fuel once you get the car running the cool fuel from the tank will fix the problem.
S Best

Also if your emissions cannister is on the way out, the fumes being fed to it are fumes while the engine is running, but soon as the heat is taken away, the fumes start condensing and after awhile, once it fills, it can start to poor raw fuel back into the carbs causing poor and uneven idle. Might not be the case but I thought I should let you know of the possibility.
Neil Campbell

I am not sure what "Crypton tuning" is, so do not know if that would show up a problem. Nor, am I sure whether the UK spec cars had the charcoal cannister system that the US spec cars had in 72.

We do know that the car runs well until it is shut off when hot, at which point something fails and the car runs poorly until the engine is completely cool. Thus, some form of problem related to parts heating up. Thus, S. Best's reply about the condensor is a place to start. Inspection of the coil is also a thing to check as, if it has internal shorts, it could overheat and fail in use. As to the carbs, they need a heat shield with intact insulation to function properly. I have never seen a "vapor lock" problem with SU carbs having a good heat shield. In fact, the classic "vapor lock" is normally associated with a mechanical fuel pump mounted on the engine, not the carbs themselves. If the fuel in the SU fuel bowls is heated to the point that it vaporizes, the float should drop and the electric fuel pump should fill the bowl with fuel. So, I would focus on an electrical problem myself. Les
Les Bengtson

Les,

"Crypton" is the manufacturer of diagnostic engine equipment.

"Crypton tuning" is a generic name for hooking your car up to sensors on the coil, plug leads and exhaust pipe and testing the operation of the coil, distibutor and carburettors. The machine had an occiloscope to display the waveform of the lt and ht circuits.

It would have detected a bad condensor - but not one with an intermittant heat related fault.

UK cars don't have any of the smog equipment - including cannisters!

Our unleaded fuel is very volatile and vapour locks can push the fuel clean out of the fuel line, back past the pump so that the pump cavitates and you only get a dribble of fuel through. It is rare, but it does happen.

Happened to me once in a motorway jam - engine spat and spluttered, pulled off into a services and spent half an hour fiddling, cleaning, blowing through jets, etc. engine would start with difficulty and then run like a dog - got fed up, went in for a coffee, came out and the engine started and ran perfectly!
Chris at Octarine Services

That is a serious vapour lock Chris , I have seen the temp gauge heading off the dial on my car , usually in a mega jam , but have never suffered from boiling fuel. The cure would be to ease one of the connections near the carbs being careful not to spray neat fuel onto a red hot manifold . (If you boiled the fuel in the float chamber it would escape via the vent and new cool fuel would be pumped in per reply above , so this has to be vapourising in the pipe .)I have seen hot T Types without the aftermarket heat shield show exactly the sypmtoms you describe . Funny thing is that the same "cure " as for carb icing at the other end of the temp problem scale will work, just leave the car to settle out thermally .Of course switching off when overheating is a last resort as the water may boil as soon as the dynamic head of the pump is lost on the water in the head .
S Best

This thread was discussed between 17/04/2004 and 18/04/2004

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