Well your clutch is slipping....are you sure its backfiring? Or are you just turning the motor over while trying to start it and what you hear is the motor turning and not a backfire?i was trying to take that nut off and i dont know how. it seams to keep spinning even if i have something holging my tire in place.
While you got it running check for a vacuum leak, spray some WD-40 or carb clean around the manifold, if the engine RPM changes, then you have a vacuum leak.i already had the kill switch disengaged, and tomarrow im going to get a new plug and wire. the bike will run but VERY porley and only for a block or two. its eather not fireing right or like you said something with the carb. ill let you know how the plug and wire works