oil in combustion chamber of Honda GX25

I removed my Mantis tiller from a couple years of storage, filled it with fresh gas and, when I tried to pull the start cord on the Honda GX25 4-stroke motor, it would not turn. Eventually I found that the combustion chamber had become filled with oil. I drained the gas tank and the crankcase, removed the spark plug and propped the unit upside down overnight to clear the combustion chamber then installed a new spark plug and refilled the crankcase and gas tank. After two or three pulls of the starter cord, the motor would not budge - the combustion chamber was again filled with oil. I guessed that somehow (perhaps by being frozen) a ring or maybe the piston had been broken while in the storage shed so I tore the motor down only to discover no broken rings and an undamaged piston. There was more carbonization on the valves than I would have expected for a motor that has probably less than 10 hours of run time on it but, other than that, I see no signs of wear or damage anywhere. If anyone has any suggestions as to how to fix my problem, I would very much appreciate them.