Did you have the tank fitted to the engine?
These 'Pulsa-Jet' carbies use the intake pressure pulses, acting on part of the diaphragm, to pump fuel up to the well in the top of the tank. That well is a 'constant level' device, analogous to a conventional float/needle setup.
From there, the fuel is drawn up through the main jet by the partial vacuum at the carby venturi, as with a conventional float carby.
So it needs the pump to keep it topped up - excess fuel just overflows back into the tank. As the well/chamber doesn't hold much fuel, it will empty quickly if the pump isn't working. Also, it can take quite a few pulls of the starter to fill it up from empty, e.g. when the tank has been off the engine for cleaning/overhaul.
