Yanmar SB 12 wont reach maximum RPM in neutral
I have a 1976 Cal 29 with a single cylinder 12 horse Yanmar. The engine has about 1400 hours on it with no visible leaks or or maintenance issues. It appears to have lots of compression as it starts easily even in cold weather. The problem I'm having is that it will only rev up to about 1900 RPM. In gear I get black smoke and oily residue at 1900 and I understand that it may be over proped but this dooesn't explain why it wont rev up in neutral. In neutral there is no smoke but it will only rev to 1900. There no restrictions in fuel flow as it has a new Racor filter and clean tank. Also it doesn't feel like a fuel issue as it will pull hard right up to 1900 rpm. I have also checked the linkage and it appears to have full range of motion right back to the stopper.
The previous owner said he only ran it at 1200-1400 rpm. I can only make about 4-4.5 knots at that rpm. If anyone has any ideas I would appreciate it. Thanks.