If the plug was black, either you have just been on choke or the air/fuel ratio is rich. Check the air filter. If that is OK you may have a sticking choke, Suzuki handlebar choke levers get rust under them, (though yours looks too good for that,) cables can stick, but often it is the plunger sticking in the carb casting. If the choke is fine, someone may have been fiddling with the slow running mixture screw, or the main jet. I don't think the main jet needle is adjustable on that carb, so probably would need a new (standard) main jet and needle.