INSULATION CONSULTATION

James Turner contacted me to ask whether his Velocette’s hot starting problem could be related to inadequate heat insulation for the carburettor. He says: ‘I’m happy the magneto is sparking properly, but wonder about the paper gasket on the carb – would a thicker ‘Tufnol’ spacer be better?’

If you wash your hands in petrol, they get very cold. Evaporation has a refrigerative effect and this tends to keep carburettors fairly cool, even though they are bolted to the hot cylinder head. It also contributes to ‘carburettor icing’ – but only works while the fuel is flowing (and evaporating) in the hot carb. I’ve heard fuel boiling in carbs when an engine has just stopped – and obviously if the supply is turned off, that could mean the carb is nearly empty when you come to restart. But my guess would be that insulators are more to do with preventing excess heat at high speeds than starting problems. I’ve mentioned before that my BSA A10 still iced up, even with the carb bolted metal-to-metal to the head, and I’d guess James’ problem lies elsewhere.

Experience tells me the slide cutaway is often the issue with poor starting, but settings overlap rather, and an incorrect pilot jet or needle setting can also have an influence. James came back to say he discovered the slide needle was one notch lower (weaker) than the standard position and will report back when the weather improves.