Black smoke - all the time

[Johnnie Engesser]

I have noticed that our Yanmar 3GM30f (Hunter 340) seems to be producing black smoke at all times - at idle, at load, etc. It does seem to be slightly more dense at high rpm 3000 plus. In other posts I have noticed that the new props may contribute to the smoking problem. We installed a three blade max prop in the early spring but I can't say that I noticed a difference then. However, I do not remember seeing black smoke last year. Our prop has been set to allow the engine to reach 3400 rpm per our probably not so accurate tach. On the other hand we only reach a boat speed of 5.6/5.7 knots - fairly disappointing. I assume from this that we are not over prop. What to do about the smoking?

 

[Bryan C.]

Things to check

Lots about smoke in the archives, but check & clean your air filter first. DIrty prop wouldn't cause smoke at idle. If your air filter is clean most likely problem is a clogged exhaust system, the most likely culprit being the U shaped mixing elbow. Black smoke means too much fuel is getting burned for the relative amount of air.

 

[Gordon Torresen]

Black smoke

I concur with Bryan C. These are certainly areas to check. Your 340 must be relatively new so "coking" may not have happened. Your top speed of 5.7 and the ability to turn 3400 could indicate that you don't have enough horsepower. Your prop has been set accordingly. We know that the 3GM engine has been powering the Hunter 34's quite well for a long time. Now, if you have one cylinder that is not firing, you would have reduced HP AND black smoke all the time, even at idle when not a lot of air must pass through. You should have this checked out. If my guess is right, getting the third cylinder on line will get the HP where it belongs and do away with the black smoke. Your boat speed will not improve until you reset the pitch on the prop. You should then see about seven knots.

 

[Bryan C.]

Check compression levers

Good point Gordon. Possibly a compression lever is open, but it would probably run worse than you are describing if that was so.

 

[Aaron]

What about an engine problem?

I don't know a whole lot about boat engines, but if they are like any other normal one you could be leaking oil. That can cause a loss of power and lots of smoke. The fact that it is being produced at all times makes me think that some oild is getting past the rings on the pistons. You might want to check and see if you have been using more oil lately.

 

[Bryan C.]

Oil smoke is blue. *

nm