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Old 06-27-2009
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Yanmar - black smoke in exhaust

Calling all diesel experts...

I'm not very knowlegable about diesel engines but I have read a number of posts on 'black smoke' so far on this site and others.




I have read the basic symptoms from a number of sites but http://www.catalina320.com captured it quite well:
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 [2] - Air intake restricted [3] - Exhaust restricted [3] - Leaking inlet or exhaust valves [4] - Poor engine room ventilation [4]
Black Smoke Excessive Fuel:
Causes: Defective injector(s) [1] - Incorrect injector nozzle [3] - Injector pump incorrectly set [4] - Low fuel grade [4]
Black Smoke Engine Overload: As load increases the governor senses the slight decrease in RPM and adjusts the injector pump to deliver more fuel. If the engine becomes overloaded then the increase in fuel does not increase RPM and no extra air is being sucked into the cylinders. Net result - soot:
Causes: Boat bottom dirty [1] - Propeller dirty/fouled [1] - Engine alignment incorrect [2] - Propeller incorrect [3] - Transmission incorrect ratio [3]
So here are my specifics:
1986 Pearson 28-2 which I've owned for 2 years. No problems thus far.
Yanmar 2GM20F, original engine, unknown hours, ran well until today
Last fuel filter change in fall before hauling for winter

Symptoms today:
Max RPM I could achive under load was 2700 (that would only drive the boat at aprox 4.5 knots), normal max rpm under load is over 3100 rpm (which will normally drive the boat at just over 6 knots)
At 2700 RPM thick black smoke would come out of the exhaust, if I backed down to 2500 RPM smoke went away and speed woud drop to 4 knots or so
Plenty of water in the exhaust
Back in the slip, in neutral I could max the RPMs out at 3400 or even higher with no black smoke

Other info:
Not had this problem before and there has been no change to prop or other engine components.
I can't imagine the bottom or prop are fouled since it was cleaned, scraped and painted just before launch on April 15th or so of this year.

I'll be having a mechanic take a look at it but was wondering what troubleshooting steps and/or questions to ask the mechanic that some of you might suggest.

Anything jump out at you?


Thanks in advance.
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~Jim
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1986 Pearson 28-2

Last edited by JSL3; 06-27-2009 at 06:30 PM. Reason: added more detail
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