1981 Nissan Van engine hesitation spluttering

My 1981 carberated nissan urvan is having a problem when under power to the extent that it canot be driven at all. While parked and the accelerator is depressed the engine splutteres and coughs with only enough power to build up the revs slowly but it is able to keep the revs with no problem its only while trying to accelerate, it even starts perfectly. I thought it might have been the fuel pump so i have replaced the old mechanical pump with an electrical pump and have checked flow and pressure.