Why do you suspect the fuel pump? Is the fuel line not holding pressure?
It operates on crankcase vacuum, so more RPMs means more pumping power. If the diaphram gets worn or perforated, it could get to where it needs the high RPMs to keep the carb bowls full.
If the fuel line is holding pressure, I'd be more inclined to think misadjusted carb float, not closing the float valve, flooding the motor. If gas drips out the carb throats when the motor is tilted full up and you pump the primer bulb (engine not running), that's what it is.
If it's not that, I'd be more inclined to take another look at the slow speed orfices to make sure they're squeaky clean, and then see if the slow speed mixture needles need any adjustment.
