if you are turning plugs black in a hr your jetting is too rich. causes are main jet and needle obviously, but a really dirty filter will cause a rich issue as well. you may also want to clip the plug wires back about a 1/4 of a inch. the caps screw off. sometimes they get corroded and stop sending as much voltage. you also need to think about smaller domes if your compression gauge is correct and the bore/rings are good. 120 is starting to get down where issues may pop up.