Compression check cold vs hot.

Joined
May 4, 2012
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67
What is the difference between a cold vs hot compression check?

Will one give higher numbers than the other?



Ran a check on a mid 80's Johnson 70hp, and got 95-100 on the top and bottom and 75- 80 in the middle. This was cold, and it was cold out. 40 degrees and windy.



The thing ran good, so I took the chance and bought it.
 
Joined
May 4, 2012
Messages
67
Re: Compression check cold vs hot.

I'll have to double check my gauge (that was the first time I used it) but I think I was actually at 95, 80, 95.

Oddly enough the middle cylinder went higher when I cranked it a 2nd time, but the top and bottom did not.
The jumped right to 95 and stayed there. The middle jumped to 75, then 80, and may have moved a tiny amount on the 3rd cranking.


I hope that the variation, is OK, and that the lower numbers is because it was so cold, because this is my motor. No money to change things up again.
 

JB

Honorary Moderator Emeritus
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Mar 25, 2001
Messages
45,907
Re: Compression check cold vs hot.

That middle cylinder is just a little out of spec (+/- 10%) as measured, tiller, but I do not trust cold measurements if a warm measurement is possible.

Could just be some coking that would go away with a good decarb. If she runs, do a decarb treatment, then run the test again as described in the Top Secret File ^^.

Good luck, and let us know what happens. :)
 
Joined
May 4, 2012
Messages
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Re: Compression check cold vs hot.

I was a terrible buyer on this one.

When I stopped to look at it the other day I was in a big hurry. Basically could look at it long enough to hear it start up and run.

Then same story when I bought it. Checked it out long enough to do the compression test, then bought it.



It's actually a 75 Evinrude 70HP. And from what I gather and can find it has a power tilt kit, but this isn't tilt/trim. Any ideas on this? It has the trim motor on the outside of the engine bracket, and I'm pretty sure it only has once cylinder that lifts the motor-that tells me it's a power tilt kit add on, and not power tilt/trim.



In any event I bought the rig cheap enough, and it runs good enough. Once I part the boat out, and sell the trailer I'll only be in it for a couple hundred bucks most likely.
It has one
 
Joined
May 4, 2012
Messages
67
Re: Compression check cold vs hot.

It's actually a 75 Evinrude 70HP. And from what I gather and can find it has a power tilt kit, but this isn't tilt/trim. Any ideas on this? It has the trim motor on the outside of the engine bracket, and I'm pretty sure it only has once cylinder that lifts the motor-that tells me it's a power tilt kit add on, and not power tilt/trim.


:confused:
 

jbjennings

Captain
Joined
Jul 18, 2007
Messages
3,903
Re: Compression check cold vs hot.

THe 2stroke motors I've tested show a couple of psi lower when warm compared to cold. I would try a decarb, but it sounds to me like the middle cylinder may have a problem like some pretty bad scoring. It may run a long time that way and run smoothly, but I would think at one time or another it either overheated or ran lean. I would surely rebuild the carbs on it and make sure that middle carb is flowing plenty of fuel. I believe you could even have a friend drive and look in the carb throats with a light and see how the middle carb is flowing compared to the other two and see if there's a really obvious problem, but a rebuild of the carbs would be the thing to do either way.
JMO,
JBJ
 
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