To understand this problem, you also must know that there are different color smokes that a lawn mower can blow out the exhaust pipe. Blue, black and white smoke are the colors that you would normally see and in this article we will concentrate on black smoke.
Black smoke means that the engine is running rich. Running rich means that the engine is getting an incorrect fuel/air mixture. It is getting more fuel than air and thus burning in a rich condition. A clogged air filter is often the culprit here.
Black smoke is normally a rich condition. In most cases the engine is getting too much fuel to burn and cannot burn it all thus making the black smoke. The engine could also not be getting enough air. Things that can cause this is an air filter that is very dirty or a setting in the carburetor or fuel injection that is incorrect.
Try removing the air filter and inspecting it for dirt buildup or grass clippings. If it is clogged, purchase another filter and install and then test the engine. Carburetor or fuel injection settings should be performed by a qualified lawn mower repair technician.
Each lawn mower engine smoke condition has its cause and by knowing what color to look for and the steps to take to correct it, you can cure the my lawn mower is blowing black smoke problem.