It's running lean, not rich. The main jet is at least partially blocked - possibly as simple as a drop of water in the float bowl. Drain the carb and fuel line by unscrewing the bowl nut. Forget about the drain screw - it's not at the bottom of the bowl and can leave water at the bottom.
If this works, great. If not, you will need to strip the carb.
phil_saunders@bigfoot.com
Phil Saunders, September 2008