1977 evinrude 85 hp bad compression in 1 cylinder

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Adam Olson

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Jul 6, 2004
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I brought my 1977 85 Hp evinrude into the local boatshop cuz it was running a little wierd. He checked the compression on all the cylinders 3 of them were above 100 but 1 was a 40. he recommended purchasing a used or rebuilt powerhead. From what I have seen on the net, that runs upwards of $2000. My question is: It runs ok. it can be hard to start, but I have taken it out a couple of times and I can get some good speed out of it. SO should I invest the 1500 bucks or is there an easier fix, or can I use it the way it is for now without causing serious damage><br />Thanks for te info<br />Adam
 
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walleyehed

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Re: 1977 evinrude 85 hp bad compression in 1 cylinder

I'd say try a good decarb before making any quick decisions. Several manufacturers of decarb out there, Motor tuner, deep creep by seafoam. They all have their own product name.<br />Do the decarb 2 times as listed on the can, then mix a bottle of straight seafoam in your gas tank at the ratio recommended on the can and go run a full tank through at a relatively high RPM for most of the tank-full...then do another comp check and see if we've gained some back.
 
CATransplant

CATransplant

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Re: 1977 evinrude 85 hp bad compression in 1 cylinder

If one cylinder is as low as 40 psi, I don't think decarbing is going to do the job, frankly. I'd pull the head on that side and see what's going on in the bad cylinder.<br /><br />It's not such a good idea to keep using the engine, either. Let's say a ring is broken. A chunk of broken piston ring in the crankcase is not a good thing.<br /><br />My old 35hp had compression of about 85 when I got it. I decarbed it and it went up to 105. But 85 is a lot different from 40.<br /><br />I say pull the head and have a looksee. If you see a scored cylinder, then it's off to the rebuilder.
 
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walleyehed

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Re: 1977 evinrude 85 hp bad compression in 1 cylinder

Then by all means, pull it down and look...<br />If it was a broken ring, I don't "think" it would be running as good as it does at top-end.<br />If the cylinder is not scored substantially, the head gasket will be another 18.00.<br />That plug would have some amount of metal in it if something broke in or near the piston.<br />Check the plug with a flashlight, then run a magnet over it....see what ya get.
 
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79chry

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Jun 22, 2008
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Re: 1977 evinrude 85 hp bad compression in 1 cylinder

Ok not sure how to get new thred started so i'm forced to piggy back this one. I have a '77 70hp Evinrude w/compresion in order of 100,95,90. The little i know that this isn't that bad. My problem is that yesterday was the third time i took it out and it ran like poo!!! The time before it ran ok seemed to have trouble getting up and going but eventualy would. The first time i took it out it ran fine for the most part. Does the compression have to do w/how it is "not" running???
Thanks dan
 
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