problem with fuel return

ever since i bought my car i have had the same problem where if i tried going up too steep a hill or go too fast (anything above 60) the engine would start to cut out like it wasn't getting enough gas. i put a "new" set of carbs on it and checked out the filters and all seemed to be ok but the problem never stayed away long. Finally my father clamped off the return line to the front filter and for some reason it seems to be working. is there something that could be missing from the system because i know the car had been messed with by its previous owner and we could be missing something obvious.