[C320-list] Low speed under Load

[Donald Lawson]

All
I have 2004 320 with a Yanmar 3YM30 fitted with a 15x9 MaxProp by PO.  Everything has worked well for 3 years. At last spring haulout I had the prop serviced and bottom painted. Boat worked great. Recently
I get 5 knots at 2000 rpm, which is normal.  However there is no increase in boat speed past 5 knots up to 2800rpm.  Cant' get above 2800 rpm. Rpm to 3000+ easily in neutral.  No visible exhaust  at 2000rpm. 
Skipper says some minor black smoke at 2800 rpm. Seemed like a mist to me.  Recently serviced.  Got fuel and air OK. Had diver check MaxProp.  Seems OK. Diver cleans bottom and prop once a month.
Anyone had a transmission trouble with a Yanmar? Starting to grasp at straws.

Don Lawson
"Mandolin Wind"  
Ventura, CA