Low compression 6 hp 2 stroke mercury, bad idle

Michawilden

Petty Officer 3rd Class
Joined
Apr 4, 2015
Messages
96
Hi All,

I have an 6 hp mercury 2 cylinder 2 stroke. serial number 09573702. Motor was bought with the note that it would not run on low rpm. Tested is, cleaned out the carb and no avail.
Compression test gave 90 psi on both banks ( pretty low in my opinion), spark is good.
In order to check for scoring in the cylinder i pulled the exhaust baffle plate and the side cover. Did not see any damage to cylinder or piston but I saw what I believe was a broken ring. I can clearly see, in one picture a gap in the ring in an exhaust opening in the upper cylinder. I did not see it in the same position in the lower cylinder. But I saw the same for the lower cylinder on the bypass side.
My question now: Are the ring gaps supposed to be in this position? My gut tells me it is not correct and I have broken rings ( hence the low compression)

https://imgur.com/a/7F3b2XY

Next to that I discovered the 2 lower reed valves are rusted away. This could also explain the bad idle rpm.

I can buy the rings and the reed valve but I have searched and some people also have 90 psi on both cylinders and have a fine running engine. So do I need to tear it completely open or are the rings correct and do I only have to change the reed valve?

I do not mind tearing it open as long as I know that the rings are 100 % certain broken.

https://pasteboard.co/IBb15DP.jpg
 

flyingscott

Admiral
Joined
Apr 8, 2014
Messages
6,374
That motor needs to get torn down. The ring cannot have a gap anywhere near a port. I would also take a good look at the crank and make sure all the bearings are there. And I believe your reed vales are stainless steel.
 

Michawilden

Petty Officer 3rd Class
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Apr 4, 2015
Messages
96
Oke , thanks for the response. I will pull it apart and check for damages. Will keep you guys posted as soon as I know more.
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
13,575
Mercury Marine service manuals state that for anything below 120 psig,(200 rpm cranking rpms, good battery and plugs out) expect problems. I'd say you are long past that data point and agree with flyingscott.
 

flyingscott

Admiral
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Apr 8, 2014
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6,374
Those motors will run fine with 90 psi they are not a high compression motor. It is the broken ring and the way the reed valve looks that is concerning.
 

QBhoy

Rear Admiral
Joined
Mar 10, 2016
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4,726
I think you are slightly low at 90 psi. But I also happen to know that anything but a real good and in calibration gauge can read wildly low or high to the reality.
was the engine warm or cold ?
you might find another 10-20 psi with another gauge...good tcw3 oil and a warm engine ?? And it’s a carb issue ?
 

Michawilden

Petty Officer 3rd Class
Joined
Apr 4, 2015
Messages
96
Rings are not that costly but I will have to check the cylinder if it still looks oke. If I see scoring it will be a different story.
Compression check was with a warm engine, carb open. Spark is oke, Carb is oke. It is clean, the problem is in my eyes the reed valve and or compression / rings
 

flyingscott

Admiral
Joined
Apr 8, 2014
Messages
6,374
Mercury Marine service manuals state that for anything below 120 psig,(200 rpm cranking rpms, good battery and plugs out) expect problems. I'd say you are long past that data point and agree with flyingscott.

What manual is that? I have never seen one of those motors with that kind of compression. Before you order rings find out which block you have it will either be a chrome bore or Mercosil.
 
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