Air filter is blocked so there's not enough air input. There is a problem with motorcycle sprocket, like idling air inlet congestion, then the air input just isn't enough. The oil amount of float chamber is too high, some gasoline go directly into the cylinder without atomization, that may cause incomplete combustion. Main orifice wearout seriously and then this fuel injection from primary nozzle increases, that may cause incomplete combustion. Heating automatic valve is at start position along with the enrich valve can't be closed automatically, that can caused a wealthy mixture. Carbon deposit in muffler, exhaust jam, waste gas stay in cylinder, that can caused incomplete combustion then black smoke.