[lincpimp]

Depending on the carb model that may be the fuel float bowl drain screw. Your pics and description are crap, BTW, a better pic would be nice!!!!

If it is the mixture screw then what drknow said is true. I would replace the plug anyways, and when you do check the spark condition by attaching the plug wire to the plg then grounding the threaded portion of the plug on the enfgine case. Turn it over and see if it has a decent spark. Most small engines use a generator to create spark and a fowled plug will cuase no end to running issues. Even cleaning the plug is a waste, buy a few new ones and keep them handy.

Do you drain the fuel out of this during the summer? You may have a stuck fuel float that is causing the carb to not shut off the fuel flow to the carb which can also cause a rich condition. A carb rebuild kit may not be a bad idea.