sounds to me you either have (carb starter chamber) problem or your valves need adjusted. Question is how many miles and has the bike sat for a long time. recomemd 93 octane with 5 tablespoons of carb cleaner liquid. if this doesnt do the trick check which cylinder by exhaust the one thats backfiring oull the plug wire while running if your still hearing abnormal exhaust good possibility your valves are out of adjustment.