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Old 06-10-2024, 11:53 AM
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Location: Northampton, UK
Cobra Make, Engine: Shell Valley Daytona, Ford 302
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Originally Posted by Tommy View Post
There are three common heat related fuel problems. Vaporization is when the car sits for a while and heat vaporizes fuel in the float bowl. That problem is solved when the engine is cranked over long enough for the fuel pump to refill the bowl. . . Percolation is when conditions are hot enough (usually after engine shutdown) to boil the fuel and push it out of the float bowl into the throat of the carb and down into the intake. This can create a a flooded condition on engine start AND empty float bowls. . . Vapor lock applies only to the fuel pump (mechanical or electrical). As the pump sucks fuel in from the supply line it reduces the pressure and if the fuel is warm enough it will vaporize. Fuel pumps don't move vapor well so little to no fuel will go to the carb. The problem can be solved in the short term by cooling the fuel pump and supply line. It can also be solved as described by Brian and Tom by allowing fuel to flow through the pump continuously with unneeded fuel being sent back to the tank. The problem can also be solved by placing an electric fuel pump upstream from the mechanical pump in a cooler location. It can apply pressure to fuel going to the mechanical pump so that fuel is less likely to vaporize even at higher temps.

This smorgasbord of problems and causes is why diagnosis is so helpful.
Thanks for that detailed and clear explanation Tommy. Appreciate it. Personally I think the symptoms fit vaporisation. If I switch off and immediately try to restart it will. If I leave it say 5 minutes then I have to churn it. Iíll try the opening the throttle 1/3 then to the floor technique and see what happens. I should add that when starting from cold after a long period of standing in the garage my technique is to switch ignition on, churn for about 5 seconds, pump throttle about 3 times. It will then either fire or start. If it doesnít start 1 or 2 more pumps of the throttle usually does the trick. Having a mechanical fuel pump I assume that the carb needs priming first. My roadster with an electric fuel pump is a completely different kettle of fish. 3 pumps of the throttle even after prolonged standing, switch ignition on press start button-bingo!
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