RE: 90SP Muffler failure on run #4.
I don't know what the oil content is in CoolPower 10% airplane fuels actually. I just know that this engine seems to like this fuel.
If yours is dying in transition from idle to full throttle then you have a carb tuning problem most likely. Reset the needles back to factory initial start-up specs and try adjusting them again.
Regards.
- Michael