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Vapour lock/ heat soak

My Lomax has a starting problem when hot, especially in the present heatwave. No problem from cold, she just does not want to go when hot.There is no apparent reason for this, the plugs are not even getting wet after prolonged bursts on the starter. I am wondering whether this is a familiar problem?
Some horizontally opposed carburetted engines with the long induction tract , eg, BMW motorcycles, VW beetles, some aircraft engines seem to suffer from heat soak/ vapour lock, anybody know whether this is the cause of my problem, and what to do about it?

Membership No. 1966

Re: Vapour lock/ heat soak

This sounds very like a fuel pump problem - i.e. worn non return valves in the fuel pump. This is an issue with cars that are not used every day. Try it with either a "known to be good" fuel pump or gravity feed from a can (be careful with this).

The pattern fuel pumps (probably all of them these days) seem particularly prone to this problem with the poppet valves and I got fed up of fitting a fuel pump as an annual service item. When the supplier told me I was being unreasonable asking for a refund on my third pump in two years I decided to fit an SU electronic pump. Had no such problems since. Electric pumps are more suited to cars not driven every day - much kinder to your battery and starter motor too. You don't have to pay SU prices - there are plenty of electric fuel pumps on eBay for under £20. Just make sure they are specifically for carbs and work at 2 or 3 psi.

Re: Vapour lock/ heat soak

It's worth checking the coil. Easiest way is to slot in a different coil when it is not starting. Failing coils always seem to give problems when they are warm.

Membership No. 1159

Re: Vapour lock/ heat soak

That was my first thought and it is the failure to start when hot that makes the coil a prime suspect. It may well be playing a part, but there's also the total absence of petrol.

Does the fuel pipe go near the exhaust manifold, the fins, the oil cooler or any other heat source?

Re: Vapour lock/ heat soak

Thanks, chaps, I shall follow up those suggestions. The fuel lines are in quite a hot area. As a matter of interest, with the conventional contact points system without a distributor, how does each coil know when to produce a spark for its dedicated cylinder?

Membership No. 1966

Re: Vapour lock/ heat soak

It doesn't... it is a waste spark system.both plugs spark each time -one on combustion and one on exhaust...

Membership No. 1336