Hold on to your hats, its time for the dumb question of the day. I liken the arm on a mechanical fuel pump to that of an old timey well pump. The arm goes up and down and liquid is forced out. What is going on inside a mechanical fuel pump that allows only the correct amount of fuel to make it in the carb? If the float says no more, what retards the fuel supply? Thanks for your patience. :beer
Let me guess: Its just a diaphram inside, probably rubber. My guess is the volume of gas pumped is constant, governed by the size of the chamber/diaphram in the pump. The faster the cam turns, the faster it pumps. The float in the carb would cut off the fuel supply - so the fuel being pumped would find the path of least resistance through the return line between pump and gas tank. I think of the return line as similar to the over-flow on a kitchen sink. I suppose the size of the needle valve itself makes sure only so much fuel enters the carb per second, so the return line is probably always in use to some degree. Of course a technical answer would be cool.
