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