Thank you for the suggestions I'll try to change the servo and check for loose conections this servo is not too old and it was working fine in a plane, like I said only happens when the engine is running so may be the conection is bad, I hate to take it a part since it took me almost 8 hours to fit the mechanics in the body but I rather do that to see it die before the engine is broken in. Well thank you I'll post the results soon.