Circulated Mercury

Compression test for outboard motor?

I recently ran a compression test for my 85 horse Mercury outboard motor. The motor is a 1973 2-stroke and the readings with this tester were from top cyl. to bottom – 80, 80, 90, 55. I was wondering what kind of problems to expect. This motor was run for approx. 1-2 minutes without water circulating and I know the impeller is shot but I am really worried about the motor. Thanks for the help.

Low compression on 1 cylinder does not always indicate internal damage. Carbon build-up in the ring grooves can prevent the rings from expanding, causing low compression readings. I would suggest you get a can of Ring-Free, or similar de-carboning agent, spary it liberally in all cylinders, and let the outboard sit over night. Then check your compression again. If the #4 reading stays low, then you do have internal engine damage, such as a scored cylinder, broken ring, or damaged piston. The good news is that all internal engine parts are still available for this model, including oversized pistons, so repair is possible if you decide to restore your 850.

BTW, I would not advise you to disconnect a plug wire while the engine is running. This is a sure way to blow your switchbox, and new ones are about $700.

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