Does my engine have a problem? Or is this normal behavior?

l008com

Senior Chief Petty Officer
Joined
Jun 14, 2007
Messages
751
I've got a 1989 Mercury 90hp outboard, 3 cylinder. It's been on the family boat since the mid 90s. The way I use the boat changed significantly around 2009 or so. When I went from using it to drive high speed around a small calm lake, to driving it at much slower speeds in and around larger rivers and Boston harbor.

The boat starts up and idles pretty well, though it's usually very smokey. Smokier than most old boats. But it clears up once it idles for a while.

The boat goes at headway speed just fine on calm waters. And of course this motor always runs great when you're planed out. It wants to scream. . .
But in-between is where the trouble is. Almost everywhere I boat now is no-wake zone but much of it is industrial and/or just huge area where small boats like mine won't literally go headway speed. Ironically the speeds we do go make the biggest wakes, and if we were allowed to just go 25 mph, there would be much less wake, but that's another topic entirely.

So if I'm going day 5 to 10 miles per hour and not planed out, for an extended period of time, then when I get off the throttle to go through locks, the engine really starts to run poorly. It starts smoking again and sometimes it can conk out. The other day it started to happen but I was alone, so I put it back in gear and did very slow donuts until it died down and started running normally.

If you drive at near idle long enough, it does seem to eventually clear out. But normal driving speed is more of a high idle, pushing lots of water, not planed out.

Also if the seas are choppy, I have this same problem but much worse. After bouncing around for a while, the engine will really start smoking and it will often stall and I'll have to restart it. But once it finally restarts, it will mostly run normally.

The easy answer is that I have the wrong kind of boat and the wrong kind of engine for the kind of boating I'm doing these days. But I have what I have and that's probably never going to change.

So given that I'm driving around in an over 30 year old, 3 cylinder, triple-carbeurated, 2-cycle engine, is this normal behavior? Or is this a problem? It's been behaving like this for years and it does not seem to be getting worse.

I do fog the engine every fall and it's a 2 cycle so there's no engine oil to change. I put in good gas and I often mix in some Seafoam into the fuel. But no matter what, it seems to run the same, which is pretty good overall but with some hiccups.
 

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Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,945
If it is a premix motor, the fuel mixture will get "oily" after some time in the tank. This will cause smoke and can cause some fouling of the spark plugs.

Other than that, it sounds like it is out of tune. When were the carbs cleaned and adjusted? When were the spark plugs replaced?

Check the spark plug wires for cracks. Check the powerhead gasket for exhaust leaks into the cowling.
 

l008com

Senior Chief Petty Officer
Joined
Jun 14, 2007
Messages
751
The engine has a separate oil tank and mixes itself, i assume right before 'injection'. Also the tank is only 6 gallons so no gas sits in there for very long.

Last time the carbs were cleaned was probably about 10 years ago, I'm not sure there is any different behavior now than right after they were cleaned. But I guess a tune up wouldn't hurt.
I do periodically replace the spark plugs so I don't think those are that old.

I also do have video of it running:

Note that this was the first startup of the season and it had been fogged, so the amount of smoke here I don't think is representative of anything.
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
38,062
Oil is mixed with the gas before the fuel goes into the carburetors.----Have oil pump output checked as per the procedure in the manual.----Post compression test numbers.----May simply require the low speed mixture screws to be adjusted.----Installed a new impeller recently ??----Tested your overheat warning buzzer ?
 

The John

Petty Officer 2nd Class
Joined
Nov 16, 2009
Messages
123
I have heard (but dont take my word for it) that 2 strokes dont like running at low RPM for a while and they will build up oil in the cylinders. I have always been told that for 2 strokes you can run them at low to mid idle but you need to spend some time at WOT to really clean out the cylinders of the unburnt oil. (like I said, take this as a caution not as word).

I have a 1989 100hp Mariner (mercury) that is oil injected.
 

Dave1027

Lieutenant Junior Grade
Joined
May 25, 2010
Messages
1,080
Plus the oil auto-blend is supposed to reduce the oil at idle to 100 to 1.
 

cprodave

Petty Officer 1st Class
Joined
Jul 1, 2012
Messages
301
Is this a 6 gallon fuel tank running a 90hp engine? If so that might be good news because you will likely always be running relatively fresh fuel. Bad news is you might have relatively low Range...
Is your "good gas" E10 or (I hope not)E15? Besides the Seafoam are you adding Stabil or similar? Have you tried running non-ethanol?
 
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