Small Engines (Lawn Mowers, etc.): Engine trouble Yardman push mower, mulching mower, engine trouble

Question
I have a yardman push mulching mower which is about 10 years old. It has always ran fine but a few weeks ago while mowing it started to rev really high for a minute or two then I turned it off. After restarting it each time now it revs very slow and the revs change from barely being able to cut to a slower rev, but still much less than it should. When I move the throttle up and down it blows black smoke from the exhaust. Any ideas what it could be and what could I do to fix it possibly.

Answer
It sounds as if the air filter may be stopped up and the plug is probably fouled. The governor spring may be broken and possibly the diaphragm under the carburetor worn out. When the air filter gets clogged it causes the carburetor to call for more fuel which causes the spark plug to get fouled.