Could be caused by a bad pump inlet check valve, which in this case is a checkball at the bottom of the pump well.
With pump discharge nozzle removed, hold the pump dischage needle with your finger and pump the throttle. If the inlet check ball does not seal, fuel will bubble back into the float chamber from the pump inlet passage. Replace the pump inlet check ball. Stake the ball into the seat with a brass drift punch. Not too hard. I doesn’t take much.
Could be caused by a bad pump discharge needle valve.
check the pump discharge needle and its seat for damage or foreign matter which would prevent the needle from seating properly.
Replace the check weight needle. Install in the discharge hole and using a brass drift punch tap the check weight so that it forms a good seat. Make sure the weight is loose when done.
May be improper main jets.
Are you operating over 5,000 feet? Standard installed jets are good for below 5,000 feet. If so, reduce the size of the jets by .002.
Are spark plugs black? Reduce the jets by .002.
Are spark plugs white? Increase the size of jets by .002.
Plugs should burn a gray color.
Possible excessive choking.
Check choke linkage for binding or damage. Check for freedom of action of poppet valve on choke plate. When idling check for vacuum leaks around the carburetor and intake manifold.
Idle mixture screws might be out of adjustment.
Connect a vacuum gauge to the caburetor and turn the screws in or out to get the best vacuum.
Inspect the idle mixture screws. There should be no scoring on the tapered end.