my bike has issues with the rear head. out of nowhere the back two cylinders were fouling new ngk plugs black. then the rear two cylinders stopped working at all. recently i had the tank off to work on the plugs. wondering if i may have pinched a fuel line etc. need an explanation though about the new plugs going black so soon after instalation. i pulled the plugs , gapped them and cleaned them.. still not working. Please help!!