[Gaz64]

I'd say you need larger needle and seats.

It is quite possible that you're starving for fuel on the primary side since essentially only one needle and seat is supplying the engine.

With the carb open where the secondary main system is supplying as well, the 2 needle and seats must be just adequate.

The needle and seats could be the same size from 390-850 CFM, a 390 DP is good for a potent 4 banger etc, or restricted V8 classes.

Run the largest you can while still having fuel level control at low speed.