Yanmar 1GM10

[Billows]

On Hunter Ranger 265.Max revs achievable 2200 , black stuff in the cooling water at max revs + white vapour, drives boat at 3.8 knots.Black stuff disappears if you throttle down. 3500 revs if taken out out of gear, clear exhaust water then.No warning buzzers, starts on the button.Prop checked and clear. Diagnosis?

 

[sailorman]

dirty bottom & prop

 

[Tranona]

Although you say prop is clear the symptoms suggest not. If it has suddenly started doing this then something has changed to put an extra load on the engine. Another possibility is a blocked exhaust, perhaps a collapsed bit of hose, or at the other end a blocked air intake. Black smoke is an indicator of over fuelling - that is more fuel going in (or less air) than the engine can burn.

 

[Mel]

These engines are well known for blockage in the exhaust. Take off the exhaust elbow and you may find carbon blocking the exhaust exit on the cylinder head. Mine was so small that the hole left was only the size of a pencil !