Acellerator pump overhaul

Been having problems with my acellerator pump, At junctions the engine dies, but if i pull off the accelerator the engine picks up again, its fitted to a 2.25 using a zenith carby, tried cleaning & rebuilding a few times using instructions from a haynes manual...also put a carb kit into it, with no luck, im certain its the pump cos there aint no fuel squirting from the jet when i open the butterfly....anybody rebuilt one of these? any help greatly apreciated!. thanks Jackual.