White smoke followed by a belch of soot

[Smudge601]

Hi all. Back to my Nicholson #32.....I have an OLD Buhk DV20. She threw both rear engine mounts a while back during the fix I noticed a fuel leek from a rotten return pipe. fixed that serviced the engine at the same time and as part of that i replaced the fuel filter.

Moving forward whilst I had some white smoke before I now have more (although it deos decrees a bit after some time presumably as the motor reaches working temp). The other day i had to motor at 1/2 throttle for approx 5 hours (No wind) back to port. as I berthed I used 1/2 throttle reverse briefly to pull her up. as I cut the motor back to tick over she threw out half a dozen blobs of soot. I thought they were oil at first but got one between my fingers and sure enough it was soot (carbon black).
the guy seeing me in said " to much Fuel" and walked off.

So 2 questions. I/ does anyone have any idea if the diagnosis sounds correct and 2/ what do I do about it.

[MBWhite]

How old is "old", LOL.

You ask what to do about it, there are two answers IMHO. The first answer is to say the "proper" thing to do is remove the injection pump and send it off to be rebuilt and have the fuel flows readjusted. Now you will also need to replace the injectors as your old ones will not meter the fuel exactly how the manufacturer designed it, and if you are paying for a rebuild might as well pay a little more for it to run properly. Now you also need to have the rebuilt pump installed and properly timed on the engine. White smoke on startup can also be a sign of poor compression so you should also at least have a compression test done and possibly a top-end rebuild (re-ring and have valves reground)

OR......

The fact is that your engine appears to be running reliably and just had some neglected maintenance that you have taken care of, the issues you have mentioned might affect performance but it does not sound like anything there would affect reliability, maybe the best approach is to just spend a bit more time on your boat instead of spending it working to pay for engine work, with the exception of having the injection pump timing checked/adjusted.

Now the soot stuff, et al:

I suspect that flakey soot was from your engine simply getting a good and long workout, and this is probably some old soot that has been jarred loose from that. Quite possibly the engine has finally had time to heat up to operating temperature and run long enough to give everything a good heat soak which helps in knocking junk like this loose. The white smoke is unburned fuel (primary culprits are poor compression, too cold engine often from no or stuck open thermostat, or retarded injection pump timing) and black smoke is from a too rich mixture where there is simply too much fuel in the otherwise properly operating cylinder.

Anyway, about the only thing I would suggest doing is replacing the thermostat if it has one, and if the process for checking the injection pump timing is simple do that too but otherwise just enjoy the rest of the season.

[DeepFrz]

Run the engine at 3/4 load for several hours and see if that clears up the soot problem. White smoke could be, most probably is, caused by un-burned fuel. Having the injectors checked and adjusted might take care of it but it could also be caused by low compression or, as mentioned above, the injection pump.

[savoir]

The soot has probably come from the exhaust elbow. It would be a good idea to remove and clean it. You only need a screwdriver. Chances are that it has not been cleaned in years.

[MarkJ]

Yanmar says after long low revs run it at 100% throttle WOT for 10 seconds three times.


Dint see why it would be different with a Buhk.

[sony2000]

I would start by adding a treatment of Seafoam to the fuel to clean up the system.
Its sort of a let sleeping dogs lie, or if it ain't broke don't fix it situatiojn. Just cleaning the pump and injectors should do it.

[a64pilot]

Most all of this isn't bad info

If you don't know when the injectors were last tested and cleaned, then you should remove them and have them pop tested, cheap and easy, everything I'm suggesting isn't hard or expensive, always start with cheap and easy first before you break out the wallet.
Same for the exhaust elbow, at least inspect it for clogging, it likely is.
Now an injection pump is money, I'd do it if the cheap easy stuff doesn't fix the issue first.
Sea foam does really seem to help some, but it's not a fix, lack of it didn't cause white smoke of excess carbon.
Running one hard for awhile doesn't hurt, can help clean it out a little, but half throttle ought to be enough to get her up to temp, which is what we are after, good idea I think once in awhile, but doubt it's going to clean things up and prevent the excess fuel and carbon.

I'd bet you have dirty injectors that have led to excess carbon, and that has likely at least partially clogged the elbow.


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