Small Engines (Lawn Mowers, etc.): John Deere Sabre 1538H - 28N707, john deere tractor, crankcase pressure

Question
Hi. I'm trying to help out a retired neighbor with his John Deere tractor.  It sat all summer and he uses it only in the winter for plowing.  I got it going after I replaced the damage that the mice did (eating the spark plug wire) and got it running.  The problem is after a few minutes of warm up, oil begins shooting out of the engine vent tube and the air cleaner assembly.  Oil shoots a good 5 feet into the air in every direction like a really bad joke.  The engine sounds perfectly normal during this oil vomit trick.  I am assuming this would mean that the vent assembly has somehow failed in the engine?  Is this a normal thing to happen for this engine?  Or are the mice more ingenious than I originally give them credit for?  Thanks in advance for any help you can send my way.  P.S. I have to get this fixed soon because he plows half of the neighborhood and snow is due any time now.

Answer
I believe you have a Briggs engine...28N707 is a Briggs model number.

Did you remove the valve breather and clean it?  It is located behind the carburetor, held on by 2 screws?  You can clean it with parts cleaner.

How much oil is in the engine?  What weight of oil are you using?  It sounds like the valve breather is not working correctly causing the crankcase pressure to build up resulting in the oil squirting out.  If there is a breather tube on the valve breather connected to the carb, check to make sure it is not stopped up.
Let me know what you find.
Eric