I have a 660 I had built tuned ran great. Had the case crack a tiny bit got it all redone nee again. At a shop now it backfires alot on deacceleration. When let off the throttle when moving What causes this ?also has alot of carbon build up on the grab bar from muffler

Hi, I know it has nothing to do with the video, but I'll see a video of you fixing an electric motor.

The head bolts look stretched