The mechanical fuel pump differs in that it has a void booster section. The vacuum section is operated by the fuel pump arm; otherwise, it has not anything to do with the fuel system. During the suction (or first) stroke, the revolution of the eccentric on the camshaft puts the pump in service arm into motion, pulling the lever and diaphragm down against the force of the diaphragm spring and producing suction (vacuum) in the pump chamber.

The suction would hold the outlet valve closed and pull the inlet valve open, causing fuel to flow through the filter screen and down during the inlet valve of the pump chamber. During the return stroke, the diaphragm is compulsory up by the diaphragm spring, the inlet valve closes and the opening valve opens to allow fuel to flow through the outlet to the carburetor. The operating lever is hinged to the pump limb, so that it can move down but cannot be raised by the pump arm. The pump arm coil forces the arm to follow the cam without moving the lever. The lever could only be moved upward by the diaphragm spring. This process causes fuel to be delivered to the carburetor only when the fuel pressure in the opening is less than the pressure maintained by the diaphragm spring. This happens when the way of fuel from the pump into the carburetor float chamber is unlock and the float needle is not seated.