I am running the same engine (old type of casting) with a 3267 in cowl muffler .
The engine does not like cool power fuel at all and I have gone back to running 80/20 fuel because it is offers more cooling (more oil, castor). An enya #3 plug is good.

some rpms i get with a 13.5*8 bolly @ 9500, bolly 13.5*6 bolly @ 10500, MA 14*6 @9500 .

The engine will have better rpms with a 3255 silencer / tuned pipe.