Small Engines (Lawn Mowers, etc.): Craftsman Push Lawnmower, crankcase pressure, crankcase breather

Question
I bought a used Craftsman with 5.5 hp engine. The model is 917.388270.  It seems to be filling the oil with gas, but it doesn't smell like gas...at least it isn't strong.  I have cleaned the carb with spray cleaner, but didn't replace any the needle valve or seat.  It wouldn't start easily, but now it starts and seems to be running "normally".  I drained the oil and replaced it.  I pulled the oil filler tube and found the o-ring had slipped off.  I put it back on and seated the tube firmly.  Oil isn't coming out of there now, but I want to know if there is a crankcase breather that I should check.  The air filter needs to be replaced and I'll get a new one tomorrow.  What else might cause the gas in the oil?  Also, do you have a diagram of the linkage rod setup for this mower?

Answer
I don't have any diagrams for linkages.

If gas is getting into the oil, don't try running the engine until it's fixed.  A lot of the carburetors can't be successfully rebuilt because of the ethanol in the fuel.  It just ruins the metal.  So we replace probably 75% of the carburetors that come through the shop nowadays versus maybe 20% just a few years ago.

The crankcase breather can be found by following the rubber tube from the air filter to the crankcase.  If there seems to be too much crankcase pressure, the breather may need cleaning or replacing.  But having the crankcase flooding with gas will also raise crankcase pressure.  Let me know if you have any questions.  Thanks, PK.