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 smoke is an indication of oil that is burning in the combustion chamber. When you remove the spark plug, it will have an oily and tar looking appearance to it. It can also have a baked on oil shine to it.
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.
White smoke is normally an extremely rich condition. Sometimes it can be an oil burning issue but in most cases the engine is getting way too much fuel to burn and cannot burn it all thus making the white smoke. Things that can cause this is a carburetor needle and seat leaking and filling the combustion chamber up with fuel. This fuel also gets pumped out of the cylinder and ends up in the exhaust muffler. When the muffler gets hot, it heats up the fuel and produces more white smoke.
Try removing the spark plug and slowly cranking the engine over when the engine has cooled. Be careful as a stream of fuel may come shooting out of the sparkplug hole! You can place a rag over the sparkplug hole to catch the fuel that comes out.
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 white smoke problem.