Missus and I went for a sail on Pittwater earlier this week, the wind soon dropped to nothing so I decided to motor
home. I never run my engine ( 10hp Bukh ) flat out but I decided to give it a go. I kept an eye on the exhaust and
after about 5 mins white smoke appeared, that's OK. Another 2 mins and it started to blow a heap of black smoke.
I throttled off a bit and it disappeared, I have FTC in the fuel but what made it blow so much black smoke and how
come it disappeared as soon as I backed off the throttle a bit. Was being at full throttle just a little bit too much ???.

As an old truckie, l think, white smoke big worry, blue smoke big worry, black smoke smaller worry as the first is water, the second is oil the later fuel.

Diesels should be run at full revs at times to get rid of the soot build-up. Slow running diesels is not very wise hence charging batteries on idle is a nono.
Water leaking into the system is not good, likely engine cooling system leaking, clogged or something.
Blue smoke is excessive oil burning, broken rings, leaking valve stem seals etc while black smoke just the fuel pump badly in need of service, renew injectors, glow plugs etc, methinks.

Check if your prop is clean, it may be loading up your engine to much at high revs.

nswsailor said..
Check if your prop is clean, it may be loading up your engine to much at high revs.

That could be a point too nsw, antifouling is a little way off yet and the prop was Propspeeded but
that was about 12 months ago.

Hi Trek. No mate the motor wasn't gunned, this happened after sustained flatoutness for about 7or 8 minutes
I certainly do get a puff of black smoke if I throttle up, but it's only for a second or so. And as you say...lucky
we've got sails.

Black smoke is normally overloading due to growth on prop and hull.....easy to check, Just rev engine neutral, the black smoke should disappear , if not it would be air/fuel mix problem - normally not enough air.

When motoring around I always run it at 2300-2500rpm. This is adequate to clear the crap out of it and avoid glazing the bores.
I don't run at full power (3000rpm) as the vibration is annoying. After a good warm up I do run it at 2750 for a minute or two on most runs.

samsturdy said..
Thanks gary. It's only happened once because it was the only time I've run the engine flat out
for a period of time and it only took a slight throttle back for the smoke to disappear. No
problem with the engine afterwards or since. So I'm still thinking maybe not enough air.

Sam, I hope I haven't confused you
The best way to understand the fueling control is to start at the basics
The throttle lever determines the rpm you desire to run the engine and boat speed is the consequence of your chosen rpm.
The governor in the fuel pump then maintains your chosen rpm by responding to any rise or fall of engine rpm by feeding the engine less fuel or more fuel..

For example:
Your out motoring in a 20 knot wind going down wind (wind is assisting you)
You select 1800 rpm for 4 knots boat speed. boat moving nicely,,, no black smoke
Now you turn up wind (into the wind) and the resistance of the waves and wind put more load on the engine. This load starts to decay the engine rpm so the governor responds by adding more fuel for more power to try to maintain the 1800rpm. yet you haven't moved the throttle lever. The fuel mix ratio is now fuel richer for more power to try to maintain the 1800 rpm.
You check your boat speed and find you are now doing 3 knots so you increase your throttle lever to bring the rpm of the engine up and the boat speed to 4 knots but the increase in the throttle position makes no difference to speed or engine rpm but it has richened the fuel to air ratio to a point that you are now blowing black smoke. Your engine is now overloaded, fuel rich blowing black smoke.

Ramona.. I just have the normal mixture now. I did post some time ago about burning oil and I think it
was you that suggested FTC, so I got some and followed the instructions. The initial result was quite dramatic
but now I don't burn oil so I've moved on to normal usage. Marvelous stuff. Thanks for suggesting it.

Jolene..It doesn't take much to confuse me these days but your explanation is straight forward, thanks for that.
I don't know that I need to investigate the cause because it was a one time only event and you would be right
about engine overload so I'm not expecting it to happen again. But if it does I'll be straight on to you.

Southace..Someone on the forum mentioned some time ago ( it may have been you ) that a wisp of steam
is OK so seeing that didn't worry me so I kept going for another couple of minutes until the cloud of black
smoke appeared, then, as I said, I throttled off a bit and it disappeared. No problem since.

check engine rpm you need to achieve 10% under flight revs you will either have overpitched prop or as said earlier a dirty prop

Had the same problem with my Buhk DV20 some years ago. Black smoke on acceleration and loss of power. After looking at the prop bottom boat etc found it was due to the exhaust riser being coke up with carbon. Got rid of the carbon and resolved problem. Hope this helps

You have to take the riser off. I had to use a far bit of force to remove the carbon - screw driver, hammer and various other blunt instruments! Also the water injector was partially blocked by carbon and a reaction of hot exhaust gas, sea water and steel. Freeing the water injector from this crud made all the difference to the cooling system. I do this every couple of years.

Ramona said..

Jabberwock said..
You have to take the riser off. I had to use a far bit of force to remove the carbon - screw driver, hammer and various other blunt instruments! Also the water injector was partially blocked by carbon and a reaction of hot exhaust gas, sea water and steel. Freeing the water injector from this crud made all the difference to the cooling system. I do this every couple of years.

The blocked exhaust in this case will cause a very noticeable lack of power. Samsturdy's problem sounds more like a dirty prop now. Blocked exhaust would not allow the engine to rev and in this case the engine will run flat out.
Now is the time to take that swim Sam while the water is not that bad.

Haha 'not bad' you reckon. Me and even slightly cold water do not get on. Seriously time to
think about anti-fouling. I had it done this time last year to coincide with a PSS seal getting
fitted. I was hoping the anti-foul would last 'til it's normal October schedule, but it's not
going to happen is it.