It should not run black under normal conditions, but if you're barely taking it out of idle it could gum up as you describe. I would disassemble the carb (a good cleaning is always a good idea anyway) and while you're at it make sure the first pilot jet is gone. Then reassemble and tune the pilot air screw for good operation at idle. BTW, idle should be about 1800 RPM for that engine. Turn the pilot air screw out about one turn from barely seated, then start backing it out. There should be a "sweet spot" where the engine runs the highest RPM's and sounds the healthiest.