How to identify and fix fuel vaporisation Does your classic conk out on hot days? Sort it with this guide

Has your classic ever conked out on a hot day in heavy traffic? It’s both inconvenient and embarrassing. The blame is regularly pointed – both rightly and wrongly – at fuel vaporisation. The petrol boils in the fuel lines in the engine compartment, pushing liquid fuel

back toward the tank and leaving the lines full of useless vapour. The pump then can’t properly replenish the float chamber of the carburettor and the engine stops – or, at least, will do nothing more strenuous than tick over and perhaps rev up in neutral.

The problem is, there’s a whole host of things that can cause symptoms similar to those of fuel vaporisation. In this feature we’ve started by drawing up a checklist of likely suspects for you to work your way through first. With all those possibilities eliminated, we’ll show you how to address genuine fuel vaporisation issues.

Remember that fuel can boil in the carburettor as well in as the fuel lines, so check that the cylinderhead isn’t

running at a higher temperature than it should, for some reason. Perhaps a modest head gasket leak is displacing water from its cooling passages?

There should very often be a heat-proof spacer between the carburettor and the inlet manifold, especially on downdraught carburettors. Find a diagram in your manual to see if your particular car should have one. If it should but it doesn’t, rectify this.

Also note that fuel that’s boiling in its lines may by a symptom of some other problem that’s causing excessive engine bay heat. How’s your cooling system?

You will need Heat-shields ‘Normal’ evaporation Paper element filters Basic tool kit, rubber fuel hose, aluminium tape. The supply pipe to an SU carburettor’s main jet may be vulnerable to heat if it’s overhanging an exhaust manifold. Should a heat-shield be present? When you stop the engine when it’s hot, petrol may soon start to boil in the carburettor. This can cause sluggish hot-starting. This is perfectly normal and nothing to worry about. Paper air filters that are contaminated with oil will make the engine run badly when hot. In-line paper fuel filters are intended to be changed at your annual service.