[Gaz64]

Any accelerator pump has 2 one way valves as either a check ball or silicon diaphragm.
One check valve allows fuel into the pump, but not back, and one allows fuel into the carburettor throat.
If either one is faulty, the pump generally doesn't work too well.
In the case of Weber, we have the ability to tune the VOLUME with the exhaust valve orifice.