Black smoke coming from your push-style or rider lawn mower looks terrible, and it may annoy neighbors concerned about air quality, but it usually doesn't signify serious engine problems. More often than not, it's caused by incomplete fuel combustion, and when you leave this problem uncorrected, it can lead to starting problems. A thorough cleaning of the spark plug, air filter and muffler -- and a carburetor tuneup -- should get you mowing again.
Why Engines Smoke
The power that drives a two- or four-stroke lawn mower engine comes from explosions in the combustion chamber. Like all combustion reactions, these explosions require oxygen, and when there isn't enough air and combustion is incomplete, the fuel that doesn't burn turns into sooty carbon deposits. These either blow out of the exhaust port as black smoke or they collect on the terminals of the spark plug, which protrudes into the combustion chamber. An engine that produces lots of black smoke usually doesn't run at full power, and eventually, enough carbon builds up on the plug to prevent ignition -- and starting -- altogether.
Clean the Spark Plug
If your lawn mower has been producing black smoke and now doesn't start, you can usually get it going again by cleaning or replacing the spark plug. Pull off the spark plug boot, unscrew the plug with a socket wrench and examine the terminals. If they are coated with thick deposits, remove the deposits with sandpaper or a file. If you have a riding mower or a push mower with a four-stroke engine and you see wet oil deposits, you may have a worn oil seal. You can still get the engine running by cleaning or replacing the plug, but the smoking will continue until you repair the seal.
Restore Air Flow
Another reason for insufficient air in the combustion chamber is a dirty air filter. You should clean it regularly, but it's easy to forget to do that, and when the filter gets dirty, it keeps air out. The filter is easy to access, and the procedure for removing and cleaning it depends on your mower. Many air filters are located on the side of the mower's engine and encased in a plastic box held in place by a screw. Some filters are disposable and must be replaced, while others are washable with soap and water. Paper filters sometimes can be cleaned by blowing them out with compressed air and foam filters can be washed an reused. The muffler can also get clogged and restrict the flow of air exhausted from the combustion chamber. Remove it and clean off the soot with a wire brush or replace it. To prevent burns, allow the mower and muffler to completely cool before removing.
Adjust the Carburetor
Small engine carburetors have either two or three adjustment screws -- one is for adjusting the idle speed, and the other one or two are for adjusting the air-to-fuel ratio. When the engine produces black smoke, it could be because the low-speed screw -- marked "L" on most machines -- is open too far. Locate the adjustment screws, which are often under the air filter, and turn the low-speed screw 1/4 to 1/2 turn clockwise with a screwdriver to create a leaner fuel mixture. When you make this adjustment, you'll probably notice that the engine sounds better because it's burning fuel more efficiently.