Engine lags and stalls when initially pressing gas-Solution Found?!

The bowl vent has to remain open in order for atmosphere to push the fuel thru the low speed circuits.
If yours is clear, then I agree the pump is lazy, and the carb is not transitioning well from the transfers to the mains; it may be that the step-up piston is stuck.
Or you have a large vacuum leak somewhere;does yours have a PCV port and if yes is it hooked up?
This is a pretty basic carb and shouldn't take much to sweeten up. Unscrew the cluster and make sure the right gasket is in there; I think there are about three that come in the kit, and they are not all the same. Same with the intermediate gasket; two of those, and two or possibly three base gaskets. It's easy to mess up for a newbe. If/when you take that cluster out don't turn that main body upside down! or you'll lose the pump check-ball.