Small Engines (Lawn Mowers, etc.): 16.5 hp briggs and stratton motor, overhead valve engine, briggs and stratton

Question
Hi ive got a 16.5 horsepower briggs and stratton overhead valve engine. (vert. shaft) this motor runs good, idles up and down fine but it blows oil out the crankcase breather tube (it goes up into the air filter) i took the breather off and cleaned it, its fine. i turned the motor over while watching inside the crankcase breather area and theres a small hole in the bottem and about 20 to 30 seconds after turning it over oil started coming out this small hole. evidently its getting enough oil up into the crankcase breather area and its coming out the tube after running a bit. i changed the oil (30 wt.) and filter but that didnt help. again this engine runs good, doesnt smoke and idles and revs good.  any ideas?   thanks jonathan

Answer
Is the a dipstick oring on the dipstick?  Once in a while the dipstick o-ring will leak causing a vacuum leak.  A vacuum leak can cause a loss of crankcase vacuum causing oil to enter the air filter.
Briggs does not publish any crankcase vacuum specifications but you can use a rubber cork with a hole drilled in it to check the vacuum.  Connect a vacuum gauge and insert the cork into the dipstick tube to check for vacuum.