Compression Question

[George Solana]

I came across a ‘97 Merc 9.9 two stroke. I know it’s not an antique, but just a general question about compression that could apply to older motors.

She runs good, starts up fine, but I ran a compression check and top cylinder was 92 and bottom cylinder was 91. Motor was warmed up 5 minutes prior to compression check. The service manual says anything below 100 psi or a differential of 15% is begging for a rebuild. My question is, how true is that? Can this motor be ran safely being 10 psi below the spec?

Thanks in advance for all your great advise!

[dave-bernard]

It maybe you gauge. the 2 cyl are close. if it ain’t broke don’t fix it.

[Mumbles]

I’d be happy with those numbers as they are so close together. If the compression is below spec, the motor wont be able to make full power and may have trouble idling to.

[billw]

I am pretty sure those engines have some kind of special coating on the cylinder walls….chrome or something similar. I forget. Anyway, unless the rings are just stuck, you don’t rebuild them; you just replace the power head. So, if it starts easily and makes good power, I would suspect maybe a gauge problem.

Long live American manufacturing!

[seakaye12]

Compression testers for small engines should ideally have the pressure-release (schrader) valve close to the cylinder. Testers with the valve at the gauge end of a hose will give lower readings on small displacement engines.

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vintin

[crosbyman]

if it runs ok dont mess with it. jmho…go fishing!

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[crosbyman]

Same here…..if it runs ok dont mess with it. jmho…go fishing!

if you are worried try some decarb stuff like Merc power tune or similar Evinrude engine cleaner and measure comp. again after a while.

use the short meter pictured above not the long hose one and re-measure comp. you may gain a few PSI and be close enough to perfection 🙂 .

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• This reply was modified 2 years, 4 months ago by crosbyman.

[George Solana]

Thanks to all!

[George Solana]

Borrowed a buddy’s compression gauge with release valve close to cylinder. My gauge is a cheapie with a foot long hose, release valve on side of gauge dial. This time, top cylinder ran 96, bottom ran 94.
I sure as heck ain’t messing with it now. Amazing for a motor pushing 25 years!
Thanks again guys!

[crosbyman]

25 years … that is a young fellow

my 9.5hp 9622a Evinrude is 45 years old and runs like a top with compression in the low 70s

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