Compression check cold vs hot.

[oldschooltiller]

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.

 

[oldschooltiller]

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]

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.

 

[oldschooltiller]

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

 

[oldschooltiller]

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.

 

[jbjennings]

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