Quote:
Originally Posted by JayChicago
I have always wondered about this! Air or vapor on the suction side would definitely stop fuel delivery. But wouldn’t a bubble of vapor on the pressurized side just get pushed along into the carb where it would escape out the carb’s air vent? Wouldn’t the pump’s check valve keep a vapor bubble from backing up?
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Air bubbles or gas fumes are compressible (compare to a spongy brake pedal), while liquid fuel being pumped through the system is not compressible.
This sponginess translates to a reluctance of a fuel pump’s check valves to function as designed, often described as vapor lock, more correctly as loss of prime. A volume of vapor within the pump that exceeds the quantity of fuel to be moved in a single stroke will expand/contract, preventing fuel movement. This condition is self perpetuating, as the stagnant fuel gets hotter, creating more vapor and less movement of fuel.