I have a lot of smoking coming from my lawn mower engine and when I remove the front muffler there is oil in the muffler and leaking from the muffler pipe. What is the problem?

Let me guess - you use car oil in your lawnmower.

I would suspect the oil breather. This is usually a rectangular box on the side of the engine with a hose going to no where. Report if the air cleaner is oily too, for this is a definite sign of a bad breather. The oil breather vents the crankcase.