Low but equal compression readings on 25 2 stroke Mercury

bnjtown

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Jul 1, 2009
Messages
26
Hello Everyone,

I picked up a boat motor trailer combo the other day because it was just to good to pass up. The motor on it is a 94 Mariner (Merc). I've got a fair amount of experience tinkering with this era Merc and have owned and used hard a 92 25 for years.

For a little background this motor appears to have very very low hours, it for sure has not been touched since 2006. Shows no discoloration on the head and the prop and lower paint isn't even scratched up. Was told by the seller that they purchased it along with several other boats for a research class that never really materialized so the boats were parked in storage most of their lives.

Anyways, what would be a possible cause for the lower but equal readings of 80psi (its a cheap HF tester but it can't be that far off)? I have not actually tried to start the motor, not planning on putting much work into it if the problem is serious internal damage. I'm fine with replacing head gaskets etc, but not going to mess with a full block rebuild.


Thanks!
 

roscoe

Supreme Mariner
Joined
Oct 30, 2002
Messages
21,657
Pulling the rope to do the compression test?
Throttle opened if possible?
one plug out, or both, when testing?
Did you spray any oil into the cylinders before testing?
Press in tester or screw in?
And yes, the tester gauge could be that far off.
 
Last edited:

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,044
There is no head gasket on that motor.----There are covers on the side the block that are easy to remove to inspect pistons.----But it will need an impeller before you start it !!----Throttle does not need to be open on a 2 stroke for compression testing .
 

bnjtown

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Joined
Jul 1, 2009
Messages
26
Thank a for the quick responses - I did both a pull start check and an electric start driven check. I did not pre-load the cylinders with oil, was just a quick test when I pulled the boat in the shop before I went off to do other stuff.

I'm going to put the tester on another motor and check its reading on one that I know has good compression. If it's wrong there, then it may be the gauge, this is the only thing I have used it on.
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Book compression test process is plugs out, throttle wide open, spin engine at 200 rpm...hand crank or starter does that, reliable pressure gauge, installed correctly: "If the compression is less than 120 psig, expect problems". I think you are a "tad" past that.
 

bnjtown

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Joined
Jul 1, 2009
Messages
26
Well I think I may have a faulty testing process.. Just used the same compression tester on my motor I use everyday (25 merc) and it only registered 60 on both cylinders. They check a nearly brand new nissan 15hp and it registered 65 on both cylinders..

I'm going to go to town and find a different compressor and repeat. Hopefully that is the issue.

and I have used compression testers on a lot of small motors in the past so it was property set up etc.


Will update when I have some new news


Thanks!
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Jul 23, 2011
Messages
47,306
Harbor freight compression tester????? Seriously, get a good tester.
 

bnjtown

Cadet
Joined
Jul 1, 2009
Messages
26
Disregard this thread other than as a word to the wise to never trust a cheap compression tester.

Borrowed a good one and both cylinders are 130+/-. Same with the other 2 motors i checked.

I have another question but will make a new post about it.


Thanks!
 
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