I have one of these faithful motors in my ''78 Islander Bahama 30. Original motor with very good compression. The only issue I have is the soot that it produces at cruising RPM (2200). It covers the stern of the boat at the waterline and up for about 3 inches. I have re-pitched the prop from over to under pitched with some reduction, but not elimination of the soot. I have replaced the exhaust elbow and ensured that the air cleaner is clean (no back pressure and or lack of air). Anything that I''ve missed? If these engines are prone to produce soot, is there a non-damaging way of removing this soot from the gelcoat and paint?
On the same engine, it runs hot at 2200rpm, around 200 to 250 degree. Raw water cooled. The water pump is in "like new" condition and the thermostat has been cleaned and in good condition. The through hull is also clean, as is the strainer. Any ideas?