Hi! I'm having this small issue with my Yamaha 4600YG generator. I noticed like a small amount of fuel mixed with oil coming from the mouth (intake) of that carburetor. The generator is running fine, except this leak got me concerned. What is happening here?
jvazquez53, What you describe, in small amounts like a "no puddle", small filmy area is normal, even for gennys that do not see maximum loads. Intentionally overfilling the oil for a longer run time will make it worse. On high time 1,000+ hour engines it very slowly can get progressively worse.
When a 4 cycle engine runs, tiny/small amounts of piston ring blow-by gases will normally be generated into the crankcase air on every power stroke. The back side of the piston also generates rapidly pulsating crankcase pressure changes that make it harder to separate the oil mist/droplets from the outgoing gaseous air. Small one cylinder engines have simple, sometimes primitive vapor separator filters and floating plastic disk check valves that reliably allow only airflow outward with a filter mesh capturing the oil mist thrown off of all the moving parts inside and the blow-by vapors while gravity drains that liquid oil back into the crankcase all while the piston goes rapidly up-n-down in the bore. It can sometimes be slightly overwhelmed by sustained heavy throttle, especially at high RPM heavy loads in hot weather.
I ran my new 7.8 hour runtime inverter genny for 46.3 hours (house power) during hurricane Irma on eco-idle with occasional rev-roars to start a refrigerator, a chest freezer and the worst of all; an over stove microwave's magnetron cycling on/off/on during a 60 percent power food reheat setting. It sounded just like a power washer, with heavy loads separated by shorter rest periods. When the utility power came back on almost 2 days later I fully drained the fuel system and checked the air filter for any inhaled debris carried by the high winds, where I found something just like you did. A short length oil film emanating from the valve cover crankcase breather hose end. The thin film oil patch on the air box wall stopped at the bottom of the air filter gasket, without staining the paper filter element at all.
Moral of the story; Regularly check the airbox, especially after heavy load usage and wipe away the film without a worry.