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bmiller 26-07-2007 13:47

M12 black smoke under load
I have a Universal M12 11HP two cylinder. No smoke at start up. At half throttle no smoke, full throttle under load produces lots of black smoke and a little slick behind the boat.?????

senormechanico 26-07-2007 14:28

Has it always been this way? If just recently, the first thing I'd check is the bottom condition, especially the propeller.

Steve B.

bmiller 26-07-2007 16:10

It's only been in the water for a few weeks. Did not do it when first put in.

never monday 26-07-2007 17:19

over proped?
Dirty bottom
lazy engine syndrome?

little boat 26-07-2007 18:23

i had that once. in my case, it was because i had been only using the engine at low revs in neutral to charge the house battery and carbon had built up in the exhaust hose.

Chief Engineer 31-07-2007 21:05

Most likely Injectors...but
Check your stop cable, Check your air cleaner if there is one.

Black smoke indicates either too much fuel or not enough mixture.

What I find with us sailors is this...we start up...manoevere away from the dock...raise the sail at first opportunity....sail sail sail....come back to dock...start engine..lower sails shut off engine.....moral here? engine never gets a chance to fully warm up...injectors will foul


Originally Posted by bmiller
I have a Universal M12 11HP two cylinder. No smoke at start up. At half throttle no smoke, full throttle under load produces lots of black smoke and a little slick behind the boat.?????

Jacana 31-07-2007 21:42

Guess its just the engine telling you when its going okay if you overload ,motor will smoke and not burn all the fuel and shunt out exhaust ,back off motor till it stops smoking best to run as high revs as possible whenever using engine will keep it running cleaner giving longer life.Cheers Geoff from Aussieland

bmiller 01-08-2007 12:41

I was inquiring about new injectors from a diesel shop in OR. Instead of selling me parts the mech sent this response:

All of these lay out black smoke at full throttle and it’s even worse if the propeller does not allow the engine to reach its rated maximum rpm. Always back off the throttle till the smoking quits or the engine will wear out prematurely. 80% of full throttle is a good cruise but 90% is ok. 3600rpm max at 90% would be about 3250rpm.

That guy could have sold me some pretty expensive parts instead he gave me some good solid advice. When the time comes I need parts or service he's getting a call from me. Pretty much what Geoff said also.

Alan Wheeler 01-08-2007 13:39

That is good advice and quite correct.

Chief Engineer 03-08-2007 21:49

Another thing is calculate the maximum(theoretical) hull speed of your vessel.

I find, with a lot of my customers, they find a so-called "sweet spot"
that could be tolerable noise, tolerable speed, eliminating annoying vibrations from lockers, rigging whatever, sometimes hulls/cabins develop a resonance that can be annoying.

bmiller 12-08-2007 06:32

Time for a follow up. Hull speed should be around 6.6, the most I can get before she starts to smoke is 4.8-5.0 at 2250 rpm. Not sure of the size and pitch of the prop till she gets pulled in the fall. The boat came from TX. Would there be a benefit from a different pitch prop to get more rpm or just settle for what I have now.

Ex-Calif 12-08-2007 07:44

Is this boat new to you? Did it do this before?

I did a quick google on your engine and the cruising range RPM should be between 2600-3300 and the max rpm is 3600.

If you are only pulling 2250 at WOT (Wide open Throttle) I would suspect a restriction in the air inlet side - dirty filter, rag left in the air box or something.

Alternately you could have high drag on the boat - if she is sailing well you can probably rule this out.

Finally if the prop has been changed from original I would say the pitch is too high.

As my grandma used to say, "Go back to the last thing you effed up."

We have similar hulls and power so my experience may help. I just repropped our boat. I did some speed studies today. 2-way average approximate GPS readings.

2,000 = 4.8
2,300 = 5.2
2,500 = 5.6
3,000 = 6.0

We are probably going to run 2,300 for cruise as we used to run 2,500 at 3.5 with the old prop and 2,500 is getting noisy. I think I could probably go a little higher on the pitch and may do that at the next haul out. I'd love to get 6 knots at 2,300-2,400.

bmiller 12-08-2007 10:37

The boat is new to me as of this year. I'll check the intake next. Next time I'm taking a mechanical tach along. Kinda wondering about the electric one. Where is the sender for the tach?

Alan Wheeler 12-08-2007 13:51

What is your WLL(waterline length)?
Does your engine Rev to 3600 out of gear?

bmiller 12-08-2007 21:15

WLL 24.25
Will check out of gear rpm on Tuesday

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