When the throttle valves are opened quickly, air flow through the carburetor responds almost immediately. However, since fuel is heavier than air, there’s a brief time-lag before fuel flow can gain enough speed to maintain the proper air-fuel ratio. During this lag, the accelerating system supplies the required fuel until the proper air-fuel ratio can be maintained by the other metering systems. When the throttle valves are closed, the diaphragm return spring pushes the diaphragm against its cover. Fuel is drawn through the inlet, past the inlet ball check valve, and into the pump chamber. The discharge check ball prevents air from being drawn into the pump chamber. The moment the throttle valves are 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 pressure unseats the discharge check ball and is forced through the pump discharge valve assembly where it sprays into the primary venturi through the pump discharge nozzle. Excess fuel and pump chamber vapors are discharged back into the fuel bowl through a restriction.