When the throttle plate is closed, the diaphragm return pushes the diaphragm against its cover. Fuel is drawn through the inlet, past the inlet ball check valve and into the pump chamber. A discharge check ball prevents air from being drawn into the pump chamber. The moment the throttle valve is opened, the diaphragm rod is pushed inward, forcing fuel from the pump chamber into the discharge passages. The inlet ball check valve seals the inlet hole during pump operation, thus preventing fuel from returning to the fuel bowl. Fuel, under pressusre, unseats the discharge check ball and is forced through the pump discharge where it sprays into the venturi through the pump discharge nozzle.