If you see black smoke, the problem lies in the air filter. You need to remove the filter, clean it with soap and water, and place it back. If the problem persists, you may need to fix the carburetor. In case your lawn mower emits white smoke, it is due to the oil spilling into the wrong places. The problem will not last for long and will go away soon, without you having to fix it. If your mower excessively blows white or blue smoke for a prolonged period, you may need to get it repaired. It may occur due to a problem in the breathing tube, blown engine, combustion chamber, etc.