I replaced the sparkplug earlier this year tuning up my grandson's Mongoose 90. It progressively began running like crap. Hard to start, missing, low on power. Getting older I guess, I thought well I just replaced the sparkplug recently with the proper NGK. So I looked every where testing everything I could. Only to replace the NGK nearly last and fixing it. That is the first bad NGK sparkplug I have ever purchased that was bad so it could be. Good luck! I might start buying Nippon Denso if this ever happens again.