Mechanical Fuel Pumps: Operation
Fuel tank to pump suction stroke is positively actuated by a rocker arm or rod pulling or pushing the diaphragm to create a vacuum in the pump chamber. Delivery of fuel to carburetor is spring (against diaphragm) operated, and functions only when fuel is required. The spring also controls fuel pressure to carburetor. With carburetor inlet valve closed, fuel in pump diaphragm chamber holds spring compressed at end of suction stroke (rocker arm or rod continues to operate, but free movement of linkage allows diaphragm to remain stationary). Some pumps also have a bypass passage that continually returns part of the fuel to fuel tank. This bypass also returns fuel vapors that may form in lines or pump under high operating temperatures.