i think there was a bunch of discussion on various threads about it
i vaguely recall numbers like 2psi is enough, 6 psi is probably a lot, more than that you probably will have a problem.
i think some folks mentioned that some new rebuild pumps deliver too much pressure
the diaphragm spring in the pump sets the pressure, you could tweek that spring in a new pump to reduce the pressure
fuel is about 3/4 the density of water
the fuel pump is about 2 ft lower than the carb
to lift the static head pressure the pump needs to generate a bit less than 0.4 PSI per foot of vertical lift
about two ft for gasoline is still under 1 PSI
conclusion 2 PSI should work in all situation
3 PSI on a new pump would probably be just right, give lots of margin for wear and various real world conditions while not being too much pressure for the carb float valve.
-s
