what is wrong with my yardman tractor?

Sounds like you could at best have a plugged crankcase breather,at worst your engine needs overhauled.There should be hose somewhere on the side of the motor going up to the carb.-in case of the breather when you put the plug back in and cranked it the crankcase pressure couldnt get out.In case of the overhaul- all your compression is going past your rings and into your oil pan creating pressure through the whole engine and not allowing the piston to move

Bob R.
April 2007