why are you guys having so much trouble with grp plugs? i have owned 5 ninja engines(4-.21's and 1-.28) and the only time i have had trouble with plugs was when my tune was off. i have broken in all of the engines with the supplied #6 or #7 plugs with zero trouble. the cold plugs are a little harder to tune, but they are very tough. currently i'm running 2 ninja .21's and a ninja .28 and all are running the grp #6 plug. i have used 7 plugs total between these engines since january of this year and the plugs i threw away weren't even bad, i just felt like it was time to change based on the amount of fuel put through the engines. i will be trying the grp #4 plugs to see if the tuning window gets bigger like i've heard.