Rochester B or BC? Year of engine, etc? Was it working ok before? If the pump is working and gas is making it into the float bowl (and assuming the float is adjusted correctly), then gas is definitely making it to the main jet. I'm sure you know this already but in the B or BC, gas is sucked up through the main jet and through the crossbar over to the other side of the carb body where it goes down through a passage to the idle adjustment screw where it is metered and fed into the manifold below the throttle valve flap. If you have a loss of vacuum, then nothing works right. If you have a plugged up vacuum passage, then nothing works right.
Were all gaskets installed correctly (not plugging up any vacuum passages)? You mentioned a screen. Which screen? Are air bleed holes (top of crossover piece) free and clear? Is the idle adjustment screw out 1.5 turns? Please give us more information and we can give you some cogent thoughts.
