Answer for How does a mechanical fuel pump function ?

When the engine starts, the pressure of eccentric cam falls on its rocker arm and the diaphragm comes down. By this process, petrol comes to the pump chamber from the petrol tank through the non-return valve. When the pressure of cam is relieved from the rocker, with a spring diaphragm again rises up and the petrol that came into the petrol chamber again goes into the float chamber of the carburettor with its pressure.