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MG MGB Technical - Difficult to start when hot

My '73 mgb roadster has an annoying habit of not starting when its hot. If I stop the engine, say to fill up with petrol, it wont start until the engine has cooled down after about 15 minutes. Other than this minor glitch, the car runs perfectly.

I thought the fuel may be vaporising in the fuel pipes around the engine bay, but now I'm not so sure.

Does anybody have any simple suggestions as to what may be the problem?

Thanks in advance

Dave
David Evans

Failing coil ?
.

Could it be a tired, loose engine, ripe for rebuild? Check compression when at operating temp.I also like the idea of vapourising fuel, or an iced carb.Guess I have not helped any!
Ken R
Ken Rich

When did it last have a full tune-up? I would go though the valves, ignition (plugs, points and timing), and the carb adjustments. It is unlikely to be the coil, it would be a big coincidence if it only ever failed when you turned the engine off, but could just possibly be going open-circuit as the heat builds up in the engine compartment at switch-off. However I would expect the same to happen when stopped at lights. Easy enough to check for a spark at the plugs (when it won't start) as a first check, though.
Paul Hunt

Is fuel vaporisation a real problem or just a myth. If the petrol in the fuel line/ carb vaporised, wouldn't the fuel pump just pump more fuel while the vapour was just forced out of the overflow pipe?
Charles Goozee

Charles-
Yes. The fuel in the float bowls can vaporize if the car is left standing after a long run. Since David's 1973 MGB has SU HIF4 carburetors, the heat-sensitive mixture adjusting device of the SU HIF4 carburetors may be adjusting the mixture too lean, thus making the car hard to start. This is most common on cars which have either had the heat shield removed or if the insulation on the heat shield is in bad shape.
Steve S.

If the fuel in the bowls vapourises the pump simply refills them when the ignition is turned back on, which is why the pumps chatter more when switching on after cooling down from a hot run than otherwise. Vapourisation is only a problem when it occurs in the jet, but hardly ever occurs on the MGB, certainly not in the UK.
Paul Hunt

This thread was discussed between 25/04/2002 and 30/04/2002

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