Please let us know final out come. It would be interesting. If you have unburned fuel and injector spray patern is good, sounds like either worn piston seal or bad valve seal. A compression test ( or pressure drop test) should tell you if this is the problem. Sounds likely as when piston is compressing, leak by would allow unburned fuel into the exhaust. Could try adjusting valves- very basic procedure and instructions in the Yanmar operation manual.