You can run without the muffler, but it has to be MUCH richer, you are blowing out alot of fuel without the muffler attached, and keep it between 1/2 and full throttle and it will run waaay faster, more intake because of less restricted exhaust equals more POWER...not great fuel economy if that concerns you...alot of fields wont let you fly without a muffler other than .049...Rog