How do you fix a smoking lawn mower?

If it is a push mower there are only a couple things that will cause the mower to smoke.

1) The rings are worn and need replaced. This lets oil seep past the rings and burns the oil in the cylinder. This will cause a white smoke. (this is probably the most common cause for engines smoking)

2) The carburetor is putting too much gas in the engine. This will cause the engine to run rich and make a black smoke.

3) There is an external oil or gas leak that is leaking onto the motor and burning from the heat of the engine.

If it is a riding mower there are many more things that could cause it to smoke.