40 Hp Evinrude Compression test

Joined
Sep 19, 2011
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16
Hey guys I am brand new on here and I have a 40 HP 1985ish outboard. The battery in my boat is dead. Is it okay to hook jumper cables up directly to the positive and negative terminal wires just for the compression test? I don't want to buy a battery yet until I know how the compression is. Got the boat and motor for free so I guess I could put a little cash into the project.:D Im open to any advice!
 

fireman57

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Aug 24, 2004
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3,811
Re: 40 Hp Evinrude Compression test

hook the jumper cables to the battery terminals. don't hook them to terminals on the boat as jumper cable clamps are pretty large and you don't want to accidently touch anything else and short out things like a rectifier.
 
Joined
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Re: 40 Hp Evinrude Compression test

Thanks for the input! Will this work even if the battery is dead?
 

fireman57

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Re: 40 Hp Evinrude Compression test

If a battery is too dead you won't be able to jump it. Unhook cables from the dead battery and hook your jumper cable to them. Make sure that you have a really good connection though.
 

lmuss53

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Sep 9, 2008
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Re: 40 Hp Evinrude Compression test

If the motor sat for a while shoot some pb blaster into the cylinders and wait a while before testing. See the top secret file about awakening a sleeping outboard at the top of the page. Stuck rings may give a bad compression reading, but can sometimes be unstuck without a major teardown.
 
Joined
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Re: 40 Hp Evinrude Compression test

I tested the compression. 120 psi on to cylinder and 90 psi on bottom. What does this mean? What should I do now?
 

nwcove

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May 16, 2011
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Re: 40 Hp Evinrude Compression test

set the motor carb down, remove the plugs, bring the bottom cyl to tdc, as mentioned above give it some pb blaster in bottom cyl, let it soak, spin the flywheel a few times by hand, then retry comp test.
 

nwcove

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Re: 40 Hp Evinrude Compression test

oh..sorry, just bring the bottom cyl up to the plug hole "top dead center", if the piston is below the exhaust or intake port, anything you spray in it will just run out and do nothing. (try to make the motor as level as possible so the solvent goes around the rings fully.)
 

lmuss53

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Re: 40 Hp Evinrude Compression test

Auto parts store or walmart it's like wd 40.
 

watergecko

Petty Officer 2nd Class
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Dec 1, 2010
Messages
111
Wd40 is a water displacement chemical, not a penetrating fluid. It also leaves a film that contributes to further sticking once it dries, so no. Get pb or any "penetrating flud" at wallyworld or local auto parts store.

Just like having right tool for job, ya need right chemicals too. Would ya use a wrench as a hammer?


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fireman57

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Aug 24, 2004
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3,811
Re: 40 Hp Evinrude Compression test

let it soak for a couple of days and go out a few times a day and try to turn the flywheel by hand with the plugs out. It should eventually unstick. You might need to keep squiting pbb in there daily so it can work it's magic.
 

nwcove

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Re: 40 Hp Evinrude Compression test

quote "Just like having right tool for job, ya need right chemicals too. Would ya use a wrench as a hammer? "
ummm.....if i cant find a hammer? yes! lol

will the motor run? use some seafoam in the fuel, it could help, if it is a stuck ring problem.
 
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