I have 3 of these engines. The 1 is for sure you are using the wrong fuel. 10% or better as said above. Also you can remove the baffle in the muffler. It will increase the power allot! Some say it will cause you to lose back pressure to the fuel tank but, mine run fine. these are good little engines. Evolution claims they are broke in from the factory but all of mine needed to be broke in. I notice the more you run them the more reliable they become.