I'm having issues with my daughters 05' CRF 100, it keeps backfiring. I bought this bike used a few months back and when I tested it out then, it did the same thing. The guy I bought it from said it have been sitting in the garage for over a year. I figured bad gas, dirty carb, the fuel mixture screw needed adjusting or needed a new plug. I did all of the above but nothing seems to work. The backfiring is worse when the bike is idling. If i back the idle adjust screw all the way out it seems to run better; better not great. There are no mods to this bike, everything is stock. The previous owner did not rejet anything so i have reason to believe that the factory jets are still in there. Any assistance would be greatly appreciated.