short intermittant white smoke from prop exhaust

[kjspylite]

1989 Mariner 75 HP 3 cylinder
Serial # 0C167888
Compression was perfect: 120 across all three

After fixing my spark issue, I finally got the motor to run. It started immediately and idled pretty good. Not perfect, but far from rough. Maybe a small hiccup or two everynow and then. And it seemed a touch fast. I attributed these things to 1) it simply being a 2 stroke and 2) I probably need to adjust carbs. Nonetheless, I'm quite happy with the idle.

So I let it run for a while. About 10-15 minutes later I got a big puff of white smoke out of the exhaust and the motor stumbled a little, but not enought to stall. I checked the pee stream and it was quite cool. The water from the prop was warm. The smoke lasted about 20 seconds or so and went away. I turned it off to give it the eye ball test, everything looked OK. I started it back up and let it idle again. about 10 mins later, the same white smoke appeared. The smoke is much more white and much more in the amount than the normal small amount of grey smoke. This time I gave it some RPMs as I did not want it to stall and about 5 seconds later the smoke went away. Speeding up the motor seemed to make the issue go away. I did not see the issue again.

Also, when I gave it some RPMs, some water would come out of the poppet valve / pressure valve hole on the back of the engine (this was indpendent of the white smoke). This was only when I revved it up. I don't think its normal but it may be. I've read that the small hole is a vent for the diaphragm and not meant for water to come out.

My questions:
1) Is it normal for water to come out of the poppet valve cover, or do I need to replace the diaphragm (or possibly other parts of this assembly)?
2) Do you think the white smoke and poppet valve issue are related?
3) If not, what do you think is causing this short spurts (about 30 seconds) of large amounts of white smoke?

Pee stream was always cool and prop exhaust water was always warm. Nothing, including the engine itself, was ever hot.

Thanks for your help, this site has been tremendous to me, whether its people answering my threads or just reading other peoples threads.

 

[ONERCBOATER]

Re: short intermittant white smoke from prop exhaust

i am not sure this will be of any help, but to me white smoke rather then blue or black (either can look grey) would be steam. you said you have 120# on all 3 cylinders that sounds good to me. I am not familiar with the poppet valve you speak of, but i can not see an engine manufacturer intentionally releasing water inside the cowling, perhaps this valve releases out side the cowling? i am looking at
http://www.boats.net/parts/search/M...412GD/CYLINDER BLOCK AND CRANKCASE/parts.html
for the diagram....not sure it is right motor, but i can not see how the poppet valve in that pic would cause white smoke.
there are two options i can think of that would give you white smoke.... water somehow managing to get in the crank case/cylinders..and water splashing on the exhaust pipe intermittently. the only place i could see this happening with good compression would be at the exhaust plate section.
other possibilities that come to mind are, the white is actually blue/grey and appears white, perhaps it is loading up a bit with oil and fuel at idle, and then clearing? perhaps only on one cylinder. carbed engine can load up especially if the carbs are not synched and adjusted properly. this may also be the cause of your fast idle.
as i said before i am not very familiar with the engine, i am sure someone else will come along and provide much better responses.

sean

 

[kjspylite]

Re: short intermittant white smoke from prop exhaust

Thanks for your reply sean...

I found this thread and this is probably my cause of water coming from the poppet valve hole... http://forums.iboats.com/showthread.php?t=228858

Still unsure if this would cause any smoke from the exhaust. I'm thinking I agree with you and that it has nothing to do with it. The OEM manual shows the parts but it says nothing about how it works or what it does. From different threads on this site, I've read that it opens up at a certain RPM (or, more specifically, at a certain pressure) and relives water pressure in the block and diverts water to the exhaust. This seems to be the general consensus. I have also read that it helps warm the water before reaching certain parts of the engine, preventing cold water to contacting hot metal, so maybe the purpose of the poppet valve is two-fold.

I will try to run the motor and take a picture of the smoke if I can replicate the problem. Maybe someone can reasure me the amount of smoke and the color is normal and its just oil buring. This is my first boat and I have no experience in working with them. I have learned A TON by reading my manual and reading through these forums but I'm sure there is no substitue for experience. I might have to wait for the weekend as I have a 7 month pregnant wife and a 2 year old. Time is scarce....

Nik

 

[ONERCBOATER]

Re: short intermittant white smoke from prop exhaust

yup that link definitely sounds like the water dripping question has been answered.
i know on my lil 20hp merc she smokes a good bit on startup(cold) then once warmed and idling is fine...and a puff when accelerated after idling for a while....thinking it loads a lil or the idle speed fails to provide as much turbulence in the cases as high speed and the oil gets a chance to settle in instead of getting blown out, then when accelerated it blows out the extra oil.

Sean

 

[CharlieB]

Re: short intermittant white smoke from prop exhaust

White smake may be steam from water injestion.

When it 'coughs' and you see the steam, immediately shut it off and inspect the spark plugs, is one of them noticably 'cleaner' then the others?

If so then you may have a small leak in the gasket(s) of the exhaust plate, allowing water to be sucked back into the exhaust port of that cyl.

You may also try to inspect the piston tops thru the spark plug holes, again, if the one is noticably cleaner from the 'steam cleaning', it is a pretty good sign that the exhaust plate needs attention.