Since the electric pumps all seem to be constant-velocity, what happens to the over-pressured gas when the car is idling and does not consume as much fuel, or even when the ignition switch is switched on (so the pump is running), but the engine is not consuming any of the gas streaming into the carbs?
Quite simple - nothing. The pump will continue to run and hold the line at what ever the stated pressure is, some of the 'solid-state' pumps control their speed to do so others will have different mechanisms.
High-pressure systems used on injection engines will often have a return line which is used by a pressure regulator to dump the excess fuel while the engine as at low load. It's needed on these because maintaining a constant pressure in the fuel rail is critically important to accurate fuelling. The accuracy of the fuel pressure is far less important with carbs as the float bowl acts a a buffer.
