I have read some guys saying to change the muffler... but I cant with this plane... else I need to change the mount position and tilt it about 45 then still I think the stock muffler will touch the firewall... the unloading of the prop and the fuel shifting back makes sense which was mentioned... I can only richen it up and hope for the best... once its in the air I don't need power at all... can cruise less than 50% throttle and still have enough speed... I opted with this engine cause its so light for a 90size... which is why I am using it in that 60 size plane...