The back fire is standard for that bike and new motorcycles as well -- They are set to run lean from the EPA for environment like all cars -- To really stop it you would have to change the muffler; rebuild the carb -- Big bucks --- I have noticed that the more I drive it and more miles I get on it, the backfire lessens -- I have seen twin bikes do the same thing -- As long as it starts easy and runs good just leave it --