Gotta watch ouy for this problem i had, if youcleave the fuel valve open after use and fogot to turn it off, sometimes the fuel seeps in slowly into cylinder and will pqss through the side of piston and will start to fill up inside with the oil and will fill it up all the way, and when you go to ride it, all the pressure from the piston comming down will compress the gas and oil and it will blow out the crankcase cover seal out, thats what happened to me, i ended up making a home made gasket with a little stronger material and never had it fail for over 3 1/2 yrs now
i don't have a fuel shut off valve and has never leaked into the engine. maybe you had a problem with your float bowl in the carb sometimes it can still be open and it can leak. i had a problem similar to that but wouldn't leak into the engine but it would leak from where the idle screw port is
