With pump discharge nozzle removed, hold the pump discharge needle with your finger and pump the throttle. If the inlet check ball does not seat, 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. It doesn’t take much.
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.
Are spark plugs black? Reduce the size of jets by .002.
Are spark plugs white? Increase the size of jets by .002
Plugs should burn a gray color.
Inspect the idle mixture screws. There should be no scoring on the tapered end.
Make sure these are all clean and passages are clear.
Idle transfer holes may be clogged or damaged. Remove, inspect and clean.
Check for worn, distorted, or damaged pump piston.