Should compression hold for a certain amount of time?

About inboard or outboard motors.

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areame
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Should compression hold for a certain amount of time?

Post by areame »

I have a 1964 Evinrude Big Twin 40 hp that I brought to a shop to run a compression test. They said it hit 100 on each cylinder but only for a second and then the pressure dropped to 0. They said because it isn't holding pressure the heads are blown and I should find a new motor.

I thought it was just the initial hit that mattered when testing these older outboards. If it doesn't hold compression does that mean it's just a bad seal in the heads?

I'm fairly familiar with the lower units on outboards but haven't been into the anything under the hood, so feel free to correct me if I'm wrong.

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Roberta
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Re: Should compression hold for a certain amount of time?

Post by Roberta »

It sounds strange to me that the motor should hit 100 and instantly drop to zero. The compression tester should hold the highest pressure attained during the test. Usually 2-3 revolutions will develop the maximum attainable compression and the test guage should hold that reading until manually released by the person doing the test.

You can buy a tester fairly cheap and run that test yourself. Is the motor running poorly and that prompted you to have the test done?

Roberta :?:
Roberta "Queen of the Boat Builders"
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jenko
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Re: Should compression hold for a certain amount of time?

Post by jenko »

I agree with Roberta
first test, guage should hold pressure ,till schrader valve released or valve in the guage is faulty,
A second test with a bit of oil mist in the cylinders should tell more and yeild higher pressure indicating leakage past rings
the other test you can get done is a leakdown test.
where compressed air is put into the cylinder ,with an adapter that screws into the spark plug hole, while at top dead centre and if the head is leaking air will go into cooling system you should hear it if what they are saying is correct
in any case you need a second opinion :?
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Last edited by jenko on Fri Jun 13, 2014 9:04 am, edited 1 time in total.

gdcarpenter
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Re: Should compression hold for a certain amount of time?

Post by gdcarpenter »

Me thinks that someone may be trying to pull one on you. I was a professional auto mechanic for years and most every compression tester I used 'held' it's readings. There is usually a schrader valve in the inlet to the compression gauge, just like in your tire valve stem, that lets air in but does not release it until you depress the Schrader valve tip to release the pressure.

Don't know your engine specs, but most outboards seem to produce about 120 psi if all is tight, so at 100 psi there may be some wear on the cylinders/piston rings - enough to warrant rebuilding - can't say.

There are testers that send compressed air into a cylinder to measure the leakage rate, and help you determine where it is leaking, but they are not common. No cylinder is going to 'hold' it's max compression pressure for any significant amount of time.

Me thinks a second opinion might be well advised.
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areame
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Re: Should compression hold for a certain amount of time?

Post by areame »

Roberta:

I had the compression test done because the first time I started the motor it wasn't pumping any water (ran it for about 30 seconds until I noticed no water coming out and shut it off). The second time I ran it, after I replaced the impeller which looked ok, it wouldn't start at all so I was a little worried it had overheated.

I'm going to pick up a compression tester after work today and try it myself. My uncle is in town who has done rebuilds on a few 1950's Mercs and he's going to look at it with me. I'll let you all know what we find.

Thanks for the replies everyone!

gdcarpenter
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Re: Should compression hold for a certain amount of time?

Post by gdcarpenter »

Can't imagine 30 seconds hurting anything. Maybe 'twas just a plugged 'spit hole'.

Do a 'dry' compression test, note results.
Do 'wet' compression test (after squirting oil into cylinders), note results.

Numbers of wet test should always be higher than dry test as oil helps seal rings to bore. More familiar with automotive but say 20 psi or so increase normal, the bigger the jump the more worn the rings/bore are.

All you need is 1) Compression 2) Fuel 3) Spark. - simple eh! Good luck.
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cusoak
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Re: Should compression hold for a certain amount of time?

Post by cusoak »

Sorry this a little late, but hope that you bought one that screws in to the spark plug hole.
The other thing you will want to record is the difference between cylinders. You want them all to be close to each other. If when you put some oil in and it makes a big difference that can be a sign of worn rings or scored cylinder.
Does it still have spark.
Jeff

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areame
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Re: Should compression hold for a certain amount of time?

Post by areame »

Sorry for the delay on this. I brought the outboard to a different local guy for a second opinion and he was able to get it started without much trouble. Something with the carb was stuck/wrong and it wasn't getting any gas. Heads are fine!!

gdcarpenter
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Re: Should compression hold for a certain amount of time?

Post by gdcarpenter »

Thanks for letting us know, glad it worked out for you.
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areame
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Re: Should compression hold for a certain amount of time?

Post by areame »

Thanks, me too.

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jenko
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Re: Should compression hold for a certain amount of time?

Post by jenko »

glad you worked it out , maybe you shouldn't revisit the person who origionally diagnosed it :?

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areame
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Re: Should compression hold for a certain amount of time?

Post by areame »

Yeah it's funny, the first place I went to was a dealer with a showroom and nice website. The second guy, who knew what was going on, was just someone I found through Craigslist who worked out of a pole barn. I'm really happy with the last guy, he took pictures and documented his whole process, and his pricing was fair.

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Lowka53
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Re: Should compression hold for a certain amount of time?

Post by Lowka53 »

:? a dealer wants to sell you a new engine not repair older ones no money in doing that :roll: :wink:
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