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sandycohen 05-28-2011 05:20 PM

Ink black exhaust water - soot?
I bought a 1979 pearson 32 about three months ago with a Yanmar 2gm20f. Until last week I ran the engine at around 2200 rpm cruising. Then I realized it wants to be run higher and recommended speed is 80 percent of max. So, the last time out I cranked her and all was well. Today went out and after five minutes noticed black stream in the exhaust water (not smoke). It may have been there from go but I didn't notice. When idled down no black. When run under high rpm with no load (neutral) no black. I took a sample of the black water and it is not oil, it's sooty. Engine doesn't seem unusually hot and gas who be fine. Don't know about filters, haven't checked.

Is it possible that the two hours max i ran engine at 2000 cruising over a couple of months built up so much soot in the elbow or muffler that i just need to blow it out by running at high (correct) speed? I didn't persist since black stuff coming out the back the boat freaks one out.

Bottom is clean and i don't think there's a prop issue (it's folding) and there is no rough running, trouble starting, speed fluctuations etc..


bubb2 05-28-2011 05:49 PM

It is unburnt diesel. Chances are you are over prop'ed. To much pitch and the engine can not reach full RPM and burn all the fuel you are trying to feed it. Try adjusting the folding prop, back off on the pitch.

sawingknots 05-28-2011 06:09 PM

bub is right,the black soot is caused by overloading the engine,yanmars are famous for this,what makes you think it needs to run at a higher rpm?throw the tach away and use your better judgement,higher rpm's should only be used in an emergency situation and only for a very short peroid

sandycohen 05-28-2011 06:45 PM

1. Per the original post, is it possible I am just blowing out carbon buildup in the system from my low rpm runs? Is it dangerous to the engine to run with the black exhaust water to test the theory?

2. Everywhere I look online says Yanmars DO want to be run at 80% of max output which would be 2720 - are you saying that is wrong??

3. The PO installed the folding prop and didn't have this problem.

bubb2 05-28-2011 07:04 PM

Sandy, soot comes out of a engine as black smoke. I have a 2gm20f in my Beneteua. Your problem is one of two things, fouled prop or to much pitch. This is why you are only seeing the problem when you put the engine under load and not in neutral. Soot is black smoke, unburnt diesel is black ink in the water.

sandycohen 05-28-2011 07:55 PM

Bubb - Is there anything damaging to the engine about having black ink in the water, other than to the environment?

Am I correct that it is necessary to correct the pitch, rather then just idle down and achieve the same speed, because then i am running too low an engine speed which will cause soot and black smoke?

The stuff in the water is black particulate - is that what unburnt diesel looks like?

Guess I better read up on what pitch is and how I adjust it!

saillife 05-28-2011 08:05 PM

It is a good idea to run the engine 75 - 80% of max. Diesel engines like / want to be run.This is what Mack Boring will tell you if you take one of there maintenance classes. In fact the guy teaching the class suggested going out once a season for a couple of hours and running the boat like a powerboat. The issue for us sailors is that a lot of time the engine never gets a chance to run at operating temps long enough to stay 'clean'. I know that my engine only runs for about 20 minuets when I go out and when I come back.

bubb2 05-28-2011 08:09 PM

Dive on the prop, it could be barnacle growth. But if the prop is clean then I would play with the pitch. I am not going out on a limb and tell you that this could not be damaging. What I do know is if it is not taken care of it will get worse. BTW, when I am motoring I am at half throttle or about 2200 rpm. That seems to be the sweet spot for my boat where the engine is the smoothest and I get the best fuel consumption 1/4 gal an hour. The difference in speed is 5.4 knots compared to 6.2 knots at 3/4 throttle and using 1/2 gal of fuel an hour.

sandycohen 05-28-2011 08:11 PM

Bubb - You have a 2gm20 as well?

I have an off topic question that another owner could answer - the oil dipstick (on the engine, not the tranny) has a huge distance between the high and low marks - like almost the entire blade of the dipstick - is yours that way too? It seems hard to believe that the high and low acceptable range could be so far apart unless the resevoir comes to a very narrow point where the dipstick goes, or it curves in a crazy way.


bubb2 05-28-2011 08:16 PM

I just keep it filled to the top mark and don't worry about anything else.

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