I would suspect the carb installation. If not done correctly, it can introduce fuel into the crankcase. I had this happen to mine in my 5th wheel before I sold it. I had a mechanic friend that told he could replace the carb for me. Bottom line the generator ran pretty well, but it filled up the crankcase with fuel. I took it to Onan and they replaced the carb a second time. It was perfect after that.