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 Catalina/Capri 25/250 Sailor's Forums
 General Sailing Forum
 Outboard Engine Compression Test		  New Topic  Topic Locked
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redeye
Master Marine Consultant

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3476 Posts
Initially Posted - 11/27/2012 :  11:28:13  Show Profile
I'm no brain when it comes to engine repair. Anyhoo... I've got an older outboard for sale and I've had a request to put the compression reading on the craigslist ad.

Any secret to running a compression test? Get the tester, remove the sparkplugs, and screw in the tester and crank her over some.

Read the gauge.
Ray in Atlanta, Ga.
"Lee Key" '84 Catalina 25
Standard Rig / Fin Keel
Edited by - redeye on 11/27/2012 12:04:36

binky
Navigator

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USA
115 Posts
Response Posted - 11/27/2012 :  12:02:34  Show Profile
The added rubber hose from the block to gauge increases the "volume" of the cylinder, thus less compression. A push-in, rubber style nipple to spark plug inlet---direct and short--to gauge "should" give you a more accurate reading.
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islander
Master Marine Consultant

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USA
4018 Posts
Response Posted - 11/27/2012 :  15:32:25  Show Profile
Also either run the engine before taking the reading or put a little oil in the cylinder first and pull the cord a few times to get oil on the cyl. walls. This will give you a good seal between the cyl. walls and the rings.
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GaryB
Master Marine Consultant

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USA
4303 Posts
Response Posted - 11/27/2012 :  19:07:30  Show Profile
Binky's is correct. Get the push-in type. You have to press it tight against the plug hole!

If it's elec. start you only want to turn it over a few revolutions while holding the tester to the plug hole. If it's a manual start you will have to get someone to pull the rope while you hold the tester. Have them give the cord a good long pull at the same speed you would while trying to start it.

Run the test several times for each cylinder and use the average.

Read you're owners manual or a service manual. I think you have to ground the spark plug wire or the plug to the block when the plug is removed from the cylinder you're testing or you will blow the power pack. Not sure about this so get some professional advice before doing anything.

Personally, I wouldn't buy a tester just so I could give someone the readings on Craigslist. I don't think they are cheap. You might be able to rent or borrow one from your local automotive store but if it's been dropped or mishandled it might not be accurate.

Good luck with your sale.
Edited by - GaryB on 11/27/2012 19:09:20
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rrick
Captain

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USA
261 Posts
Response Posted - 11/27/2012 :  19:35:46  Show Profile
If you pull start, verify there isn't a decompression device. On my '05 Suzuki, you'd disconnect the cable and try your best manually turning over a 300cc 4-stroke.
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redeye
Master Marine Consultant

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3476 Posts
Response Posted - 11/28/2012 :  04:30:01  Show Profile
Thanks guys that helps me a lot. And my fav advice... Read the Manual... something about grounding the plug or burning out the magneto was the issue I was worried about. Nice tip on oiling the cylinder or run the engine... Thanks Islander.

<< I wouldn't buy a tester just so I could give someone the readings on Craigslist. >>

Yepper.. that kinda gripes me also...
Edited by - redeye on 11/28/2012 04:33:15
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GaryB
Master Marine Consultant

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USA
4303 Posts
Response Posted - 11/28/2012 :  09:09:45  Show Profile
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by redeye</i>
<br />Thanks guys that helps me a lot. And my fav advice... Read the Manual... something about grounding the plug or burning out the magneto was the issue I was worried about. Nice tip on oiling the cylinder or run the engine... Thanks Islander.

&lt;&lt; I wouldn't buy a tester just so I could give someone the readings on Craigslist. &gt;&gt;

Yepper.. that kinda gripes me also...

<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">
I wouldn't do it. Usually when someone starts getting that specific they will end up being a PIA after the sale. If the motor isn't perfect (like new) after they buy it they may come back to you saying you falsified the compression test just so you could sell the motor or want you to pay to get it running perfectly.

Compression test are iffy anyway. You could run a compression test one day using one type of compression tester and then take it to a shop the next day and get completely different readings because the shop used a different type of tester or a different method.

If you're confident in the motors condition agree to take the interested party out for a spin on the water or agree to meet them at a repair shop of their choosing. A water test should give them a good enough idea of how the motor runs. If that's not possible let the <b><u>professional</u></b> shop check the motor out while everyone is present and at the expense of the interested party.

If they don't like these options there will be someone else coming along shortly that's not so picky.
Edited by - GaryB on 11/28/2012 09:11:39
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vipermagic
1st Mate

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USA
69 Posts
Response Posted - 11/29/2012 :  15:04:02  Show Profile
Is the guy non-local? If he can physically come look at the engine, I'd offer to let him do the compression test himself. I've bought a questionable car, snowmobile, or motor or two, and always always prefer to do the compression test myself, to make sure its done how I like it. Theres obviously a thousand opinions on the issue!

Also, if I asked for a compression test and found out it had been done on a warm motor, or ESPECIALLY with oiled cylinders, I would be FURIOUS. A compression test is an excellent diagnostic tool and definitely reads to the overall health of the engine, and a hot motor will lead to questionable results at best. Oil in the cylinders will mask an unhealthy engine.

If the motor hasn't been run in some time, the honest way to do it would be to let it run and warm up completely, then cool completely, THEN do the compression test.
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redeye
Master Marine Consultant

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3476 Posts
Response Posted - 11/30/2012 :  05:03:06  Show Profile
Thanks Viper.. That sounds very reasonable. With 4 outboard engines it's good to have an understanding. Might be testing them all at some point.
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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.