Had a Briggs & Stratton 5.5 HP engine dropped off to fix as it "wasn't running right". There is a rubber hose that runs from the valve cover to the air filter housing, assuming it does some sort of crankcase ventilation. The air filter is soaked with oil. What is causing the oil to come into the air filter housing? Worn oil rings on the piston? Worn valve guides? Some other part that is clogged? The spark plug has a fair amount of carbon from running rich but is not oil contaminated. The top of the piston has carbon deposits but nothing excessive.