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3 Cylinder 2 stroke Compressions??

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gkm24

Regular Contributor
I have a 3 cylinder mercury blueband engine with the 2 carbs. Just doing some adjustments and measures before the season starts and could do with a few pointers please.

Compressions are:
No1 110psi No2 130psi and No3 120 psi. I can't find the correct figure in my manual so not sure what they should be. These are engine warm. There is no thermostat in the engine and the temps next to the spark plugs are
45 degree, 45 degree and 42 degree, 1 2 and 3 respectively. Would this engine prefer a thermostat or are the temps ok? The outside temp was 8 degrees for info.

The engine serial is 9176891 and according to data i found makes it a 1983???
Just tuning it up so any pointers welcome.
 
Serv manual for my 3 cyl 90 makes a comment about below 120 you COULD experience problems. So many things come to play in compression specific number per se. So they and most agree go for 10-15% for a range of each other.

On temps, you are less than 10%. Engine water passage design, combustion efficiency, plug perforance, fuel ratios per cyclinder can all play into it.

Sounds like what you need to do is to put the tools down, get the companion of choice, and go out and do some boating.

Mark
 
I do intend to put the stat back in as I tend to find that the engines run better wit it in. Does anyone have a figure for compressions at all?
Thanks for your comments so far.
 
The most important thing is even compression !-----With your values I would take the intake bypass cover off to take a look at pistons and rings.----Cost you a gasket to do that.
 
Thanks for that. To be honest, i think that this is the beginning of the end. It was struggling to idle in gear. I have made adjustments and got it to run fairly well and it idles in gear now, so may get one more season out of it.
 
In about 40 years of outboard repairs I have never looked for compression values in a manual.-----They are not published either.---Too many variables in gauges and techniques.----------When an outboard will not idle it is often serious internal problems.-----Do yourself a favour and remove the intake cover to inspect pistons and rings.----The exhaust cover removal will also let you look at the pistons and rings on the hotter side.
 
I agree with racerone,remove the intake cover and have a look at #1. It's the difference between the 110psi and 130psi that flashes at me.May be as simple as a sticking ring.My old beauty, 4cyl '69 Merc800 is 120psi down the line on a 30 year old compression tester and hits 130psi on a almost new tester.
Has your engine got the water cooled carb? If so I would remove /block the line as I have read that these triples need as much cooling water to the #1cyl as possible.Jeff will probably be along to tell you that soon.
 
gkm24 said:
I do intend to put the stat back in as I tend to find that the engines run better wit it in. Does anyone have a figure for compressions at all?
Thanks for your comments so far.
Click to expand...

I told you my serv man says 120 is the start getting concerned mark. But I also said since so many things are a variable, even numbers, like some use 10% my manual uses 15% between cylinders really is what matters. However, if you are down around 90 and have accurate readings I wouldn't expect any kind of stellar performance from your engine.

Mark
 
racerone said:
In about 40 years of outboard repairs I have never looked for compression values in a manual.-----They are not published either.---Too many variables in gauges and techniques.----------When an outboard will not idle it is often serious internal problems.-----Do yourself a favour and remove the intake cover to inspect pistons and rings.----The exhaust cover removal will also let you look at the pistons and rings on the hotter side.
Click to expand...


Right out of my Merc/Mariner Serv Manual P/N 90-830234R3 Dec, 1997
 

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Shut off the water going to the fuel pump--the power head needs all it can get. And do NOT use a t-stat--Merc dealers removed them for a reason.

My rebuilt triples run 140 pounds compression, but I've had good runners with 120.

Jeff
 
fastjeff said:
Shut off the water going to the fuel pump--the power head needs all it can get. And do NOT use a t-stat--Merc dealers removed them for a reason.

My rebuilt triples run 140 pounds compression, but I've had good runners with 120.

Jeff
Click to expand...

Jeff my man. Elaborate on the pecularities of this engine if you don't mind. Really interesting differences in run of the mill Mercs. Like the remove the stat dealer procedure for one.

Thanks,
Mark
 
VERY hard to remove the t-stat since all those stainless bolts in the cover have to come out--and many will snap off on you. Also, you'll need new gaskets to put it all together.

So....do the block off of the FP cooling circuit--that's easy.

Jeff
 
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