My father an I have about a dozen of these engines between us. the "bump" is normal for this type of ABC engine. Run 2 tanks at a rich setting and it will fly great. MAKE SURE YOU LOCKTITE THE BACKPLATE SCREWS IN ! They have ALL come loose and will go dead stick. TT has been very much behind there warranty, I sent two of mine in after 2 1/2 years and they repaired them with no questions.