Depending on the color of the smoke that your lawn tractor engine is putting out, there are a few possibilities of what is wrong.
Blue smoke: Blue smoke is usually an indication that oil has leaked into the combustion chamber. If the mower was tipped to the wrong side, oil can leak into the chamber. The other thing to check is if you accidentally put 2-stroke gas (mixture of gas and oil).
Black smoke: Black smoke is usually an indication that air to gas mixture has too much gas (or is too rich).
White smoke: White smoke is usually an indication that there is too much oil in the motor.