I recently purchased a Craftsman rider with a 12 hp Tecumseh OHV
model 143.386012..I purchased the mower for the deck as it was only
3 years old. The gentlemen who sold me the mower informed that the motor
was smoking and leaking oil and had not been run for over a year.
Just for fun I cleaned the carb drained all the old gas and changed the oil.
The mower started right up and smoked for about 20 seconds, it ran smoothly
and seemed to have good power so I decided to try mowing with it.
It mowed with no problems but I began to see oil dripping onto one of the steering arms. I shut it off and removed the covers.
The air cleaner housing had oil in it and oil was dripping from the head..
There is oil coming out of the breather pipe. I was under the impression that
if the rings were bad the motor would smoke all the time and this one does not..
It also seems to have plenty of power and runs smoothly..
I read in another thread that possibly the head gasket is bad..I am clueless about small engines and any help would be appreciated..
Is replacing the head gasket a big job? Could a bad valve cause this problem?
Thanks
Bill

http://www.hobbytalk.com/bbs1/showthread.php?t=248943 that thread should help you out