The pump only pumps at whatever volume the engine requires for a given throttle setting so there is not any excess fuel. For example, at idle, the pump only pumps a small amount of fuel, at full throttle a large amount of fuel. The speed of the pump is governed by the number of pressure pulses it receives from the engine through timed crankcase pressure. At idle only a few pulses, at faster throttle settings, more pulses. That is as simply as I can explain it.