Diesel Soot

[thaeni]

I have a 12hp Westerbeke and the back of my transom is covered with
diesel soot. I cleaned it 3 times this summer. What is the cause of this
issue.
Thanks

 

[Paul F]

Likely your engine is over working. Cleaning the bottom and prop will help. Also, using Power Service diesel cleaner will help.
http://www.powerservice.com/dk/

 

[Maine Sail]

Could be..

Could be many things:

Bad fuel
Over propped or over pitched
dirty bottom
dirty prop
bad fuel injector
Engine wear
etc. etc.

I have found one of the most common causes is dirty bottom and or over propped or over pitched. If your engine can't turn max rated RPM at wide open throttle, with a clean bottom, you're probably over pitched/propped.

 

[Bill Roosa]

unburned fuel

The bottom line is something is causing fuel to make it through the engine and not get burned. This is usually caused by too much fuel and or too little air.
A common misdiagnosis is a dirty bottom (or any other load like mispropping) will make the engine run slower than normal for a given throttle setting resulting in an overly rich mixture and unburned fuel aka soot. I personally don't buy that because diesels don't have throttle plates and are ALWAYS operating in the "lean mixture" mode.
However, a slightly sooty engine can coat the exhaust system and a dirty bottom will cause you to operate at higher engine output which can blow the soot out. So you see it towards the end of a season.
More likely is the injectors are not misting the fuel sufficiently (low pump pressure, injectors dirty or inop...) and causing "big" drops of fuel to be introduced into the engine which cannot be burned fully.
Anything that would cause less air to enter the engine can also cause soot.

 

[Dean Strong]

Just went through this with our local Yanmar distributor. First he says a lot of people have soot on their transoms because he is always being asked how to clean it, so not to worry about a little bit. Second, warm the engine up, and see if you can get max rated rpms in flat water. If not, you are probably over propped and that is what is causing the unburned fuel. Dirty bottom and/or fouled prop can also be a cause. But If you can reach max rpm, then you have been running the engine at too low of rpm for too long. Run the engine at max rpm for about 30 minutes. After 30 minutes, start at idle and hit the throttle pretty hard a few times. He said this would seat the injectors and clean them out a bit. Then be sure to always cruise at a minimum 80% of max rpm. A little higher won't hurt.

My Yanmar has a max rating of 3600 rpm and I have been cruising at 2800. I following the above procedure, and then have been cruising at about 3000 rpm all summer, and we motor a lot here in Puget Sound country because of summer light and flukey winds.

Just a slight trace of soot on the transom after 67 hours.

 

[kloudie1]

OK, to throw more fuel on the fire…. Uuhhhh… Diesels have a pretty crude fuel system that has almost no control follow up (electronics may cure that in the near future).. Here is the scenario.. The “throttle’ is a control that asks the governor (RPM controller) to change its setting.. so if ya have too much load, the throttle is telling the governor to go faster.. The governor turns the rack of the injection pump and more fuel is added to try to accomplish more RPM. Normally, the governor RPM would satisfy the set point and the rack is pulled back by the governor to the lean side to hold the RPM asked for by the throttle. If the motor doesn’t reach the set point, the injectors stay too rich and voila, a sooty mess.. Other folks have listed the reasons that the engine may not be able to reach the RPM set point.. all are valid