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Carburettors make quite good refigerators, not too amazing if you stop to think about it, they both work the same way, both turn a liquid into as gas. In the case of a carburettor it is a matter of more or less which might be the root of the problem.

As you turn a liquid into a gas you are faced with Boyle’s Law, (P1 . T1 . V1 = P2 . T2 . V2), (at the risk of teaching a lot of Grannies to suck on eggs). It simply says that if you have a quantity (V) of a fluid at a certain temperature (T) and under a particular pressure (P) and you change one then the others have to change too. Simply put both sides of the equation have to remain equal.

Where does this get us? Well, as the engine sucks petrol and the carburettor vapourizes it the volume increases so the temperature of the petrol vapour must drop, despite global warming, you are not going to have too much effect on atmospheric pressure so temperature is the only game in town! In fact a good deal of the petrol that gets into an engine gets there by running in rivulets along the walls of the induction system and not as a vapour at all. It improves as the enging reaches full working temperature but does not entirely stop. Don’t beleive me? There is plenty of research and film to back me up. It also goes some way to explain why fuel injection is worth the effort.

So what has this to do with Arron’s engine? Well to get the brute to fire at all the mixture has to be right. With a cold engine a bit of tickling and choke compensates for all the liquid fuel that is not going to burn. (Liquid petrol does not burn, it is the vapour that is inflamable. Sounds odd but true.) When the hot engine is stopped and then started right away the excess drops of fuel that is hanging around in odd corners has not had time to evaporate and richen the mixture beyond the useable stoichiometric ratio. If however the enging is left for a little time so that all the liquid in all the nooks and crannies has time to evaporate then you will have an over rich mixture and no start.

Is there a cure? Short of a re-design of the engine not a lot! If there is room then a thick fibre spacer between the carburettor and the engine it would reduce heat conduction from the engine to the carburettor which in turn will reduce vapourization in the carburettor. There is after all quite a bit of liquid petrol in a carburettor and even hand temperature will markedly increase the rate of evaporation let alone a hot engine. It is worth noting that the carburettor, at least on my lump, is moderately downdraught, petrol vapour is heavier than air and there is nothing to stop vapour coming up the jet tube into the carburettor choke.

The down side of a fibre spacer is that it might result in carburettor freezing but the experiment might be worthwhile as any change in the engine’s manners will tell a story.

It would be wise to check the fuel height and the effiency of the float needle.

Also turning off the fuel well before intending to stop will reduce the fuel height in the carburettor which should help.

Try draining out the crankcase oil wells with a hot engine, a petrol/oil emulsion would be an ideal reservoir.

I can’t help but think this is not the end of the story though…