I came across this on another Catalina site and thought it would be of value to our members as well. The following analysis comes from U.S. Master Marine Surveyor Rob Scanlan and is presented courtesy of RCR Yachts.
Key-numbers as to how likely the problem:
Black Smoke is caused by partially burned fuel. When the fuel/air mixture increases there is insufficient oxygen present in the cylinders to complete the combustion process. Large quantities of carbon are then produced which appear from the exhaust as minute black soot particles. Engines with turbochargers have additional sources of black smoke.
Black Smoke Insufficient Air: Causes: Air intake filter clogged  – Air intake restricted  – Exhaust restricted  – Leaking inlet or exhaust valves  – Poor engine room ventilation 
Black Smoke Excessive Fuel: Causes: Defective injector(s)  – Incorrect injector nozzle  –
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