Small Engines (Lawn Mowers, etc.): Lawnmower running rich?, dark smoke, yard man

Question
I have a 3 year old Yard Man that recently starting using more gas than normal to mow the grass.  This past weekend, it died while running and had gas.  The spark plug had lots of carbon on it, and after cleaning it, the mower ran again. Occassionally, there is dark smoke that comes from the exhaust when I there is strain on the engine.  What can I do to make it run like it used to?

Answer
Hi Rick,

A fouled plug and black smoke points to oil burning and worn rings.  Does it use oil?  How is the oil level? Is the plug correct?
To eliminate a rich fuel problem, try closing the carb mixture screw in a little at a time, if it is adjustable.  

Let me know how you make out.
Michael