It didn`t back-fire befor I redid the topend, The rings and top of the piston melted so I figured while I had it aprt I put everything new in the top, I didn`t touch anything else except for the carb got a rebuild also, and I`m breaking it in now and on the second or third ride it started the back-firing, and I also noticed a little smoke from where the exhaust pipe meets the head, could that be just a loose nut