how to test outboard compression?
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Category: General Forums
Forum Name: The Outboard Clinic
Forum Description: Anything related to outboard maintenance here....
URL: https://www.fishing.net.nz/forum/forum_posts.asp?TID=76291
Printed Date: 30 May 2026 at 5:46am
Topic: how to test outboard compression?
Posted By: Dunwurkn
Subject: how to test outboard compression?
Date Posted: 29 Feb 2012 at 7:05pm
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Hi there,
just wondering if anyone could give me some guidelines on how to test a outboards compression.
Firstly i take it you will need a certain instrument? Do the types very and any idea how much they cost?
Secondy does the motor need to be warmed up before testing or is from cold fine?
Thirdly what is the best way to go about it.
Do you need to remove all the spark plug leads?
Would you remove 1 spark plug and test that one first and put it back in before removing another or remove all three at once and and then test each individually?
When testing how long do you need to crank the motor for?
Also what readings should one be getting and what compressions are acceptable and what readings signal trouble?
Or is it better to take it to a shop and get the pros to do it.Any idea what is a reasonable price to pay for this test?
Any info appreciated
Cheers
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Replies:
Posted By: Big -Dave
Date Posted: 29 Feb 2012 at 9:48pm
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using a compression tester which is a pressure gauge with a non return valve in the line. It will either have a rubber bung on the end to push in to the spark plug hole, or a thread on it to thread it in. Remove all plugs, crank the engine for 3 or 4 compressions and read the gauge. Release the pressure there will be a tire valve or a button to push. Then test the next cylinder. |
Posted By: Big -Dave
Date Posted: 29 Feb 2012 at 9:49pm
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oh and they should all be even within 10-15 psi. 90 to 150 would be the expected ball park |
Posted By: Outboard Performance
Date Posted: 01 Mar 2012 at 12:17pm
| Any more than 3 or 4 psi on a two stroke and you have issues... |
Posted By: Blue Legend
Date Posted: 01 Mar 2012 at 2:38pm
Disconnect the ignition system before cranking
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Posted By: Unclejake
Date Posted: 01 Mar 2012 at 5:00pm
| ^ Why? |
Posted By: Blue Legend
Date Posted: 01 Mar 2012 at 5:03pm
| if you have all the plugs out, and crank the engine, wouldn't the spark have nowhere to go? maybe fry the computer and/or coil pack? I don't know for sure thats why its a question. |
Posted By: Unclejake
Date Posted: 01 Mar 2012 at 5:27pm
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Ahh, got it now.
I don't think it is a problem but shall let others who know more answer
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Posted By: Mullins
Date Posted: 01 Mar 2012 at 6:39pm
| You could put all the leads into a bucket to catch the sparks? A plastic one, obviously. Then you could tip a couple out whenever you wanted to light fires. |
Posted By: Jet_ski_fisher
Date Posted: 01 Mar 2012 at 7:08pm
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I put the kill switch on and then crack it for 2 or 3 seconds on each cyl i am at 120 all 4 ------------- http://www.legasea.co.nz" rel="nofollow">  MH... Catch measure release...<*))))< |
Posted By: Dunwurkn
Date Posted: 01 Mar 2012 at 7:20pm
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Cheers for advise did the test today 130 PSI across all 3 cylinders on my 2 stroke so all good.
Very happy with that
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Posted By: Dunwurkn
Date Posted: 01 Mar 2012 at 7:23pm
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Blue Legend wrote:Posted By: smudge
Date Posted: 01 Mar 2012 at 8:52pm
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Yes it does Chris, you should always operate the kill switch if you're cranking it over with the leads off for several reasons: a handy little fire is one of those reasons, a nasty shock is another. I know in the early days CDI ignitions didn't like an open circuit but not sure about modern systems - I'd play it safe, then there's the possibility of a big voltage hitting other electronic devices or even passing through the low voltage wiring. The insulation on the 12v stuff want stop kilovolts. The best answer though comes from Mullins  |
Posted By: Jet_ski_fisher
Date Posted: 01 Mar 2012 at 9:53pm
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How do the boat dealers do there psi checks? wonder if they take precautions on the wiring as well? ------------- http://www.legasea.co.nz" rel="nofollow"> MH... Catch measure release...<*))))< |
Posted By: smudge
Date Posted: 01 Mar 2012 at 10:35pm
| I'm sure they would. |
Posted By: Outboard Performance
Date Posted: 02 Mar 2012 at 8:29am
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On EFI motors we can disconnect crank position sensor so the motor doesnt think it is being cranked over, hence no spark generated. We can have lanyard or kill switch off and crank motor which isolates spark. You can have key turned off, then jump start starter solenoid with a wire to save getting in and out of boat to crank. We also have a special tool to plug into HT spark plug leads that earths the spark, and makes the spark jump a pre set gap so we can test the ignition at the same time. |
Posted By: Jet_ski_fisher
Date Posted: 02 Mar 2012 at 9:29am
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Question...If some one turned on the kill switch first on a 2 poke, then took off number 1 S/P lead and held it with a rubber glove and with the free hand inserted the compression tester into the number 1 cyl and cranked it 2 seconds to get compression would that do any damage even if it is not a efi engine. ------------- http://www.legasea.co.nz" rel="nofollow"> MH... Catch measure release...<*))))< |
Posted By: Kevin.S
Date Posted: 02 Mar 2012 at 12:18pm
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On an old Johnson I had turning the engine over with the spark plugs out could destroy the ignition unit. I had a steel plate with three holes for the spark plugs to screw in to and this earthed to the engine. That way I could check the compression and that the sparks were good at the same time. |
Posted By: tugboat
Date Posted: 02 Mar 2012 at 6:47pm
 what you been smoken mullins!! |
Posted By: smudge
Date Posted: 02 Mar 2012 at 9:57pm
It's not as silly as it sounds Tugboat..... if you put a fish in the bucket you could cook it at the same time
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Posted By: Unclejake
Date Posted: 03 Mar 2012 at 1:08am
| ^ You chaps do know that the phosphorescence we often see when boating at night is left over sparks from guys with loose fitting engine covers? |