I dragged out my dads old YT125 a couple months ago and fixed it up. It started on the 5th pull and ran good, then it quit. I've been through the whole thing and cant find any problems. The carb is clean, the air filter is clean, i tried a new spark plug, and it has good spark. I can only get it to backfire every couple of pulls. It has an aftermarket gold colored head, aftermarket Mikuni carb, DG air filter, and a DG expansion chamber. It would be great if anybody could give me any tips.