Im running a 72cc kit with stock exhaust and open foam filter. Ive got a 92 jet in it now, anything bigger causes popping at half to full throttle. One trick I did to dial it in was, start with a jet thats a little smaller than 120, maybe a 110, cut a 24oz beer can in half (bud light cans work the best), slip the half of the can with the open tab over the air filter and go ride it. If it pops worse, than you know your to rich. I started with a 98 jet and a can over the filter, and now im down to a 92 with no can. What plug/coil are you using?