You need to adjust your mixture screw. It sounds like it is running to rich. You can tell by the fact that fuel is going into the exhaust on the over run which ignites from the heat of the exhaust creating a popping sound. In any good repair manual should describe how to adjust the mixture screw on the carb. It may even be in the owners handbook. Always remember to adjust by little amounts as small incriments can make a big differance. If you run the bike to lean it wll pink were the extra air in the combustion chamber ignites from the heat of the piston.
Robert Whitham from R.W. MOTORS, July 2007