If you are seeing a stream of black smoke blowing out of your exhaust pipe, your lawn mower is running in a rich condition. In other words, it is getting too much gasoline and not enough air into the engine. This makes for an excessively rich condition and will eventually foul the spark plug and carbon up the cylinder.
The first easy thing to check is the air filter to make sure that it is not clogged with dirt and grass clippings. A restricted air filter will decrease the amount of air coming into the engine and can cause this very condition.
If you have a pre-filter, it can also cause the problem. These filters can be washed and reused in many cases. Use some mild dish detergent and hot water and then wring them dry and let them sit out in the sun to dry out.
The other culprit is usually a poorly adjusted carburetor or if you are at high elevations, the wrong main jet is in the carburetor. Many carburetors today have no adjustments, but you can change out the main jet on many models of carburetors.
Look at each area of your lawn mower to determine which of these is causing the problem and correct them if needed.
