OK, thanks.
Then neither the prop nor the stock muffler are the cause of this problem, which may be new, or the same one that contributed to the previous puffing (helped by underprop).

This new or agravated problem is a carburation problem on high air-fuel volume condition.

Any crankcase leak that you suspect would mess up the low rpm's, when the air intake is chocked by the throttle, and the vacumm inside the case is strong.

What is limiting the air flow? Nothing.
What is limiting the fuel flow? Only obstruction or poor tank back pressure.

If you want to verify that hypothesis before doing anything else, connect a long silicone tube to the pressure line of the tank and blow hard with your mouth into the tank while trying to rev up the engine.

I hope you have not cleaned the high end valve seat with any metal, since that may have damaged it.
If it is in good condition, clean it again with alcohol and a piece of wood.
Once the needle is out, check for surface smoothness and for good condition of any seal.
Replace the seal(s) if not sure or add high vacumm grease for extra sealing.

Clean inside the nipple at the muffler and make sure is not pointing down, since oil would clogg it during normal operation.

Reinstall and check.