"Capt John" wrote in
oups.com:

The fact that the engine puts out
black smoke, rather than heavy white smoke, suggest the compression is
good, but the injectors are probably leaking down or not spraying
properly.

Or, as this is a naturally-aspirated diesel, he's got too big a prop on it
loading the hell out of it and it's running out of air before the charge is
burned...rich. Black smoke is common on an overloaded Yanmar. The 3GM on
the Endeavour would smoke like mad if you crowded it hard....(c;