I just installed the jett muffler and noticed a huge difference, but i have a few concerns, when I fire it up and after 20 seconds of full throttle ( on the ground) The engine cuts out, does not die but looses power a alot. Should I make the high end more rich? Also the engine gets to high temps, is this because it is on the ground and not getting the airflow as it would to cool the engine in the air? I also noticed my 46ax can run on the ground at full throttle for ever and not overheat. Can someone shed some light on what is going on? the plug looks good, it is not black but when I pinch the fuel line the rpms increase which does not make sense since the engine overheats so easy? I am running a 12x4 prop with 15% synthetic fuel. Can anyone suggest a good setting from fully closed on the high end to keep the engine temp down, my temp gauge has read 270 degrees, which operating temp is 240 according to OS. Thanks for the help. I appreciate it.

12x4 is a very light prop for that engine. I ran a 13x5 on mine, and run a 12x5 on my 46AX.

Engine is getting airflow on the ground due to prop wash.

Did you pinch the line as the engine was starting to 'cut out'? The first thing I thought of was not vent pressure from the aftermarket muffler, but rpm increasing with the pinch test would say that fuel delivery is not the issue.

I'm still pretty new at all this, but I think you might be getting too many rpm with the light prop; more rpm = more combustion cycles = more heat generated.