Small Engines (Lawn Mowers, etc.): Onan motor, blown head gasket, worst case scenario

Question
John I have a Case446 garden tractor with a Onan 20hp v motor that when I start it, it will run for a minute then start pushing oil up the right side vent tube into the carb. Can you tell me what is going wrong with it and what I need to do to fix it. Anything you can tell me would be very helpful. Thank you

Answer
First, check the oil, make sure it is not on the over-full side, and sniff it to make sure it doesn't have the odor of gasoline.
If it does, then your carb is flooding while the engine is running,
and the fuel is making its way to the crankcase making it over
filled with fuel/oil, and causing your problem.
   Otherwise, your problem likely is caused by something sending
excessive pressure to the crankcase.  On OHV engines, a blown head gasket can cause this.
   A plugged crankcase breather could also cause it, and the worst case scenario, blowby, caused by worn out rings.
Take a look at these things, and see if you can find the engine's model numbers, that would allow me to look at the parts list
of the engine.
  Thanks,

  Fish