Mariner/Yamaha 40hp 1986 thermostat and spark problem, wet spark plugs

PatrickMonahan

Seaman Apprentice
Joined
Feb 3, 2016
Messages
47
I cleaned the sparkplugs of oil on my mariner and it ran great for a minute and then eventually bogged down, could this be linked to a faulty thermostat that is stuck open because the back of thge engine is coild to touch, could that affect the air fuel mixture becuase its so cold and dampen and cover the sparkplugs in oil and therefore bog the engine down? Any experience? any other reasons why the spark plugs oil up quickly and easy, was running cold today, still have to look at the thermostat, should the back of the engine be warm to touch when it is running hard ? the spark plugs are new and are ngk, thanks for your help
 

Texasmark

Supreme Mariner
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Dec 20, 2005
Messages
14,544
In every Mercury I have owned or gotten into with a thermostat, the pellet was stamped 143F. I understand that there are some engines running 120ish stats but I haven't been around them. The mid position, normal setting for a US domestic hot water heater in the home is 140F for a reference point. Since you are at 40 hp you probably don't have a high speed bypass valve so the engine block should remain at least pretty warm to the touch while operating.

Years ago Mercs had no stat and it wasn't until they decided to upgrade molds and all that, reduce the cylinder count for one thing in multi cylinder engines, give everybody a thru hub exhaust and all that that they brought in tstats. Their advertising was around the high intensity, fast rising spark to get the plugs fired before the charge could leak off due to moisture or combustion contamination. OMC (Johnson-Evinrude) had stats way back in the 50's.

No doubt that the stat is part of the combustion process. The automotive market has incorporated hotter and hotter engines in the quest for lower emissions.....aka cleaner, more complete combustion. They sell them (stats) on here....look at the top of the page under Boat Parts and Accessories.

You don't supply a complete history of your engine so any answer to your question is purely speculation. There are things that are suggested to correct normally found problems but they may not apply to your situation.

We are in the middle of winter here and cold, damp, conditions on possibly seldom used engines, especially of vintgage origin, with today's fuels, sitting around with old fuel and all are certainly a problem.

With all that said, I personally would check fuel first as the culprit while you are locating a new tstat for your engine. The fact that you said what you did tells me that you had some crud floating around and your initial satisfactory operation dislodged it from where it was and deposited it where it shouldn't be. If you never run your engine out at Wide Open Throttle (WOT) or spend a lot of time idling, or trolling, you can expect things to get clogged up, especially on cold days with old fuel. One thing I liked to run in my fuel was Sea Foam which cleans the whole system and is perfect for 2 stroke outboard engines. I ran it in every tank and kept reasonably clean fresh fuel in my tanks and just didn't have fuel related problems due to bad fuel or the like.

I think you need to install a carburetor kit and in the process do a complete clean out of it in the process. A good soaking in Berryman's chem tool, then manual cleaning of all the ports and holes, and tubes and all in the process. A thorough blowing out of all the ports with 100 psi air when finished, will certainly not hurt. A fresh tank of fuel mix with cleaning out the fuel lines in the process won't hurt. Then we can go from there. Let us know what you find.
 
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PatrickMonahan

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Joined
Feb 3, 2016
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47
Thank you for all your help. I took out the thermostat and found it to be broke in two ways, 1 that it was not closing and 2 the top part/collar on the thermostat was broke off or broke off, i have ordered a new thermostat today, what was happening was that the engine was running cold and there was oil building up on the spark plugs resultig in poor performance due to the spark plugs being covered in dark black oil , the engine was never warming up so hopefully in a week when i get the thermostat it should run properly thanks very much
 

quicktach

Petty Officer 1st Class
Joined
Aug 24, 2012
Messages
394
Also check that you're not mixing in too much oil and check spark with a gap tester if you still have a problem.
 

PatrickMonahan

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Joined
Feb 3, 2016
Messages
47
Thanks for your reply, I ran the engine with the new thernostat and it ran far hotter and smoother, it never got as hot before, there is warm water coming out of the water hole port, I got an inspection camera and had a look into the bottom cylinder through where the spark plug is and there is a thin layer of carbon on the piston head, I suspect that was something to do with it never warming up due to a faulty thermostat and idling too much, I usually use a 100:1 mixture of oil in the boat but i dont know what the previous owner used, should I run the engine hard to clean the engine out? There are some products for cleaning engines out that maybe I should try
 

flyingscott

Fleet Admiral
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Apr 8, 2014
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7,985
Carbon on the piston is normal and as long as you use an oil rated to be run at 100-1 should be fine.
 

Texasmark

Supreme Mariner
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Dec 20, 2005
Messages
14,544
Go to an auto parts store or walmart and get a few cans of Sea Foam. Read the directions on the can. For the first couple of tanks, I'd dose the fuel mix up with about 4x what's on the can. Then after things clean up and the engine is running better, drop back to the can recommendation of 1 oz per gallon of gas or premix depending on what you use in your engine. This site also has numerous posts on the subject of "DeCarb". It's a decarbonization process using SF if you are in a hurry. But if you do a decarb in your driveway, your neighbors might not like you.............makes a big smoky mess. That's why I suggested as I did. I learned about it on this site back when I joined. Decided to try it and haven't been without it since. No regrets. On my last engine I went 10 years without touching the carbs and when I did they were spotless except for a few pieces of degraded fuel line. No varnish or gum or goo or any of that, piston crowns clean as a whistle, great compression.
 

PatrickMonahan

Seaman Apprentice
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Feb 3, 2016
Messages
47
Does anyone know should this part be moving freely or shuld it be tightned by the nut one way or the other thanks very much
 
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