25hp evinrude compression question

toybuilder039

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Mar 6, 2011
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I have recently been rebuilding a 1975 25hp evinrude and when I put it back together I checked the compression top cyl was #140 bottom is #120 I no this is out of spec but was wandering if you thought it might even out after being ran. I put to new set of rings in it and also had to replace the top piston I did not bore the cylinder I thought the looked pretty good but did hone them thanks for any input. Also should it atleast run with this kind of compression.
 

boobie

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Re: 25hp evinrude compression question

Take and run it and then recheck compression after 10 hrs.
 

nwcove

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Re: 25hp evinrude compression question

...dont forget to mix it 25:1. (according to some , 15% diff is ok, yer at 14%).
 

toybuilder039

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Re: 25hp evinrude compression question

Ok I will try to get it running this weekend and I will see what happens thanks for the input
 

kfa4303

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Re: 25hp evinrude compression question

running seafoam through the motor often helps to even out the compression and improve overall performance.
 

BonairII

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Re: 25hp evinrude compression question

Your compression numbers are pretty good. A decarb will probably help. The one so-called 'low' cylinder might be caused be a leaking head gasket(among other things).

I'd def decarb and run it for a bit before jumping to any conclusions.
 

jbjennings

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Re: 25hp evinrude compression question

I don't see how it could be 140psi without a lot of carbon buildup or something else increasing compression in that cylinder. I assume you cleaned the head of all carbon deposits before reassembly and double checked the compression. I just can't see that motor making 140psi. 125psi or 130 if lucky when the motor was brand new. At least--with my compression gauge. I know they're all different. I'm just thinking there should be a reason for a 20psi difference in cylinders on a newly rebuilt powerhead. I would think that would be enough out of balance to cause it to wear out quicker. However, I'm no expert by far......
Interesting situation though,
JBJ
 
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