Carbon on the water

[crustysailor]

I have a 1980 Hunter 33 with a 2QM15 that runs fine at 1850 RPM but at 2000 RPM it puts carbon on the water.
I pulled the Mixer Elbow and it had a little carbon in it but there was no blockage.
I was told to run the engine at 1850 by the man I bought it from but that is only 7HP.
I'm told the engine should run at 3000 RPM.
It runs fine to 1900 but when I get in 25kt winds and 6ft. seas I don't have enough RPM to motor safely.

 

[Scott F. Murphy]

Sounds like you could use the operators manual:

http://www.g-o.dk/Files/Billeder/PDFer/YANMAR/Instruktionsbger/2QM15.pdf

Carbon will build up in the cylinder walls from running at low RPM. The manual suggested shut down procedure is to run the engine at idle for 5 minutes and then gun it to 2500 for a second. This blows out any excess gases and carbon. Since I started doing this, I've noticed a lot less carbon lately.
I have the original 2 bladed prop on my '79 33'. I seem to have 2 "sweet spots" for cruising, 2200 RPM's/5 mph and 3000 RPM's /6-7 mph. Between these speeds I get vibration resonance. From talking with other Cherubini 33' owners, seems to be normal.