Evinrude Compression

nopeek

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What should the compression be on an Evinrude 25? I don't know anything about the motor, it was thrown in on a deal. I am getting 90 psi for compression. The serial number is 25002E-E01881. Also what year would this be?
 

JB

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Re: Evinrude Compression

Howdy, nopeek. :welcome:

Ninety psi is fine if both cylinders compress that psi.
 
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Re: Evinrude Compression

What should the compression be on an Evinrude 25? I don't know anything about the motor, it was thrown in on a deal. I am getting 90 psi for compression. The serial number is 25002E-E01881. Also what year would this be?

I'm not sure what's worse: The person who started the 'within 10% of each other means it's OK' or
The people who think it's true.

Here's a 110 lbs piston, severely scored 360 degrees, with 110 lbs compression. Would you want 2 of these in your motor?

20130820_094526_zps0635690e.jpg


Those old 25 Johnson/Evinrudes all average about 130lbs. She's got big problems.
 
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jbjennings

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Re: Evinrude Compression

Love the popcorn photo! :)
Most of my 25hp OMC's get about 125psi (on MY gauge)---my '99 30hp Johnson had only 125psi the day I bought it and still has 125psi today and runs like a top.
I would say 90 psi is low, but the fact that they're even is what makes me suspect that your gauge may be inaccurate. The big question to me is how does it idle?
I have a mercury 65hp that I just disassembed. It had 140psi on the top, 140psi in the middle, and 130psi on the bottom cylinder. The top cylinder was fine, the middle had only 1 functional ring--the other 2 rings missing 2 inches of the ring on the exhaust side, and the bottom had 2 out of 3 rings stuck in the grooves. So, compression can definitely be misleading.
If it idles nicely I'd say your gauge is wrong. No need in jumping to conclusions since compression gauges are notorious for being inaccurate.
If you can't get it to idle, I'd check it with another gauge before I sunk money into it. Taking off the intake bypass covers and looking/feeling the rings on each piston to see that they're free and see how much scoring you have on the pistons is awfully easy and cheap. I rarely break a gasket when taking off the bypass covers.
JMO,
JBJ
 

JB

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Re: Evinrude Compression

I'm not sure what's worse: The person who started the 'within 10% of each other means it's OK' or
The people who think it's true.

Here's a 110 lbs piston, severely scored 360 degrees, with 110 lbs compression. Would you want 2 of these in your motor?

20130820_094526_zps0635690e.jpg


Those old 25 Johnson/Evinrudes all average about 130lbs. She's got big problems.

Hmm. Lemme see now: You attack the "within 10%" rule with a pic of a piston that had 110psi and you don't say what the other piston compressed. I would wager that it compressed a lot more than 110psi.

Do you agree that conditions, techniques and gauges vary and that the pressures they indicate vary with them?

Do you agree that two cylinders rarely go bad to the same degree at the same time?

Do you agree that OMC, in Service Manuals, created the +/-15% rule?

I stand by my post.
 
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Re: Evinrude Compression

Hmm. Lemme see now: You attack the "within 10%" rule with a pic of a piston that had 110psi and you don't say what the other piston compressed. I would wager that it compressed a lot more than 110psi.

Do you agree that conditions, techniques and gauges vary and that the pressures they indicate vary with them?

Do you agree that two cylinders rarely go bad to the same degree at the same time?

Do you agree that OMC, in Service Manuals, created the +/-15% rule?

I stand by my post.

Nobody is attacking anybody. The point of the 110 lbs scored piston means compression numbers mean nothing if you don't know what they are SUPPOSED to be. Is 90lbs good on a 25 OMC? No, it's not. I know that because of experience, not some general rule.

A 2 cylinder engine with the same low compression reading on both cylinders indicates a bad head gasket. That's why it happens even and abruptly. It would be equal numbers, but bad, since they fail at the bridge between the two cylinders.That's where the head warps as well.

Someone overheated the OP motor and didn't want to fix it. So they gave it away.

Save your money OP, spend on a good motor.
 
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Roberthill

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Re: Evinrude Compression

"I" would not give up on this motor yet . Have you tried running it ? Does it have fire ? If it setup a long time rings may need looseing up . Need more details .
 

boobie

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Re: Evinrude Compression

Anchor, where are there any published compression specs on OMC mtrs ?? The service manuals say no more than 15 psi between cylinders. Of course what does OMC know ?? I'll go along with JB.

PS. The last time I've seen OMC publish a compression spec was for a 1968 Snowmobile in their manual and that was 85 psi.
 
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iwombat

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Re: Evinrude Compression

Lesse, a 1970 25hp pull start. Unless you've got popeye arms, I doubt you'd get much more than 120psi on a new motor with a _well calibrated_ gauge. Yes, if you look really hard you'll find the odd motor where the 10% rule of thumb fails. That's why it's a rule of thumb and not an absolute edict. It holds true in most cases.

In the case of a blown head gasket between cylinders there will be other pieces of data to add to the mix.
 
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clanton

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Re: Evinrude Compression

The motor in question should run fine with 90 lbs of compression.
 

jbjennings

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Re: Evinrude Compression

What should the compression be on an Evinrude 25? I don't know anything about the motor, it was thrown in on a deal. I am getting 90 psi for compression. The serial number is 25002E-E01881. Also what year would this be?

When new it probably had about 125psi. No compression results were ever published because so many people foul up a simple test, misinterpret results from a faulty gauge, etc. But, it should have about 125 psi. A 25 OMC with only 90 won't idle worth a flip---if it's actually 90. Since you own it, why not pop open the bypass covers and take a peak?
JMO,
JBJ
 

boobie

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Re: Evinrude Compression

What is the compression on it and have you even tried to start it ??
 

clanton

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Re: Evinrude Compression

It will idle fine, if you turn the right screws.
 

phillnjack2

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Re: Evinrude Compression

scored pistons don't give bad compression.
its the rings and bores that give the compression !!!!
ive seen bad scored pistons get put back in old 2 strokes that have had a hone and new rings put in to get people out of trouble and last years.
so just because a piston looks scored don't chuck it away.
the piston is not supposed to actually touch anything apart from at the very bottom, its the rings that should be doing the work.

anyway, 90 psi on a old motor like that is about as low as you want to go.
its gonna run but obviously not give the power of when it was new, a touch of decarb fluid/spray might bring it up a touch more on
the compression numbers too.
was compression done when dry or with fuel in the engine ? was it warm ? etc


phill
 

nopeek

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Re: Evinrude Compression

What is the compression on it and have you even tried to start it ??

Sorry for the late reply. I got a jon boat in a trade and the guy had an old motor sitting in his shed he threw in with the boat. I'm getting 90psi for compression on both cylinders. I haven't really had much time to fool around with it too much. I have looked at it and I am going to take off and clean the carb and change the sparks. It turns over like it wants to crank.
 

jimmbo

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Re: Evinrude Compression

I often wonder how anyone can claim 120 lbs compression in an engine that has a compression ratio of less than 6:1. At sea level air pressure is 14.7 lbs. Taking a volume of air at 14.7 lbs, at which a compression gauge would read Zero, compressing it to a volume of 1/6 of what is was, the max pressure should be 88.2 lbs, not accounting for leakage at the rings. One could argue that the 14.7 static pressure needs to be added , but even then that is only 103 lbs. Most two stroke outboards have less than 6:1 for compression
 
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