FasterTHENyou

stock motor? Use a stock ignition

The previous owner could have cut the head. What carbs are you running, is it ported? Check the cylinder wall clearance. All these problems could/would lead to low compression. What bore is the motor, that could also be a problem As far as the backfiring it could be your ignition or it could be rich/lean. Lastly a fouled plug will run fine at an idle and cut out on the top end.

save it for something that will be more beneficial then an 80 dollar part.

Just looked at your profile. There really is no use for you to go to a +14. I have a +14 on my 800cc banshee and now im going to a 12 for more hook. For your motor id stick to the 12 with a bar for the track or if you want to improve your 60's go with a 10. BTW, what track did you run the 3.8 on and what was your 60' and mph

what motor are you running? There is no reason you need more then a +12