The Yanmar 2GM20F in my 1990 Hunter 30 is driving me crazy.
In 2007 the mixing elbow coked up, I pulled it, cleaned it, and reinstalled it. Since then, the engine starts lightly smoking (gray) after about an hour at cruising RMP (2850-2900, 80% of max rating), and leaving an ever present soot stain on the hull after of the exhaust
At idle there is always a slight soot discharge visible on the water surface (on return to port).
I've run tests to make sure the 3 bladed prop is the right size. I get 4100 rpm in neutral, and 3750 in trials, achieve hull speed of 6.7 knots.
At the end of last season, I had a mechanic pull and rebuild the injectors, and adjust the valves. At the same time I pulled the mixing elbow again, it was half blocked, and replaced it. Also replaced the air intake filter.
The engine runs at normal temperature--I can always touch the mixing elbow even after hours of hours running, our cool Northwest waters keep it just warm to the touch.
The engine runs well and consumes very little oil between annual changes. I had the oil lab tested this spring and all is normal. The engine has 700 hours on it. I never idle for prolonged periods or run at low RPMs; always at 75% to 80% of max rating.
Its not the elbow, not overheating, not injectors, not burning oil, not overpropped, but still emitting unburned fuel
I think about just running it, but I know the elbow is getting clogged, and slow but steady damage may be occuring to the pistons and cylinders.
Diesel experts, please tell me where to go next with this.