6HP Mercury compression (issue?)

I've got a 1986 Mercury 6HP short shaft that was rebuilt about a year ago and used lightly since then with varying results. Some days it will blow your hair back, and on others it will not even stay running once given some throttle. It always is a little rough to start in the morning, but starts on one pull after running for a few minutes. I have done a lot of work on it since then including fuel/carb/spark/coil work and have not tested it in the water yet since it's winter. I just bought a compression tester after waiting for a while, which brings me to the real question at hand for those "experienced" motor guys. I tested both cylinders (70/75 PSI) dry, then dumped a little motor oil in them and rechecked (140/145 PSI) both cylinders. I know the cylinders were slightly scratched before the rebuild, and pistons and rings were replaced as well as new gaskets all around. I just feel this is low compression for any outboard. Am I now looking at oversize rings or worrying about nothing at all? Thanks in advance guys!

compression seems good to me. $.02

That's good to hear. I have seen a few others here (and on the street) say that the oil trick means bad news. Hopefully I'm in good shape. Thanks Friend!

If you had to put the oil in there, the compression is bad. I would take it back to whom did the work and talk to them. See what they say.

It will always measure higher with oil, that doesn't suggest a problem. If there was a compression issue, it would never perform well. Since performance varies, I think the issue is still in the fuel/spark realm.

I was just cruising iBoats website on some other topics and started to feel REALLY dumb. I found some info on compression testing. I realized that my first test (low readings) I left one spark plug in while the other cylinder was being tested. The second test (high readings) was with both plugs out for the whole test. I have a strong feeling that this rookie mistake is skewing my real results. Any thoughts on this besides the fact that I was in too much of a hurry to focus? Thanks again Gents!

Both plugs out, throttle wide open, give it 2 -3 pulls on each cylinder.

Just got a few minutes to go back to the basement to try it again (uninterrupted this time).
I did each cylinder three times and got an average of 75-80 on both cylinders.
I'm on the fence about the compression now.

If it were mine, I'd lay the engine down so the spark plug holes are vertical. Squirt a small amount of oil in each cylinder and slowly turn it over a couple of times by the flywheel or starter rope. This will help coat the entire cylinder with oil.
Stand it back up and do another compression test.
If the readings make a big jump you could have a problem with ring seal, which could be scored cylinder walls, excessive clearances, out of round bore, etc.

That's the same test I did before. Exactly the same way I saw the compression hit 150. I guess the cylinders have a few issues. I'll check it again after running it once the weather warms up some. Thanks!

if its a pull start, i would keep pulling until the needle maxes. mine takes 6 - 8 pulls.

[url=https://www.tinboats.net/forum/viewtopic.php?p=337571#p337571 said:

JoshKeller » 31 Dec 2013, 19:24[/url]"]if its a pull start, i would keep pulling until the needle maxes. mine takes 6 - 8 pulls.

Click to expand...

^^^ this!

Your compression is always going to go up some from dry to wet testing, but you're wet test is getting almost double what dry is and that to me says you have issues. I have the factory service manual here somewhere for that motor. I'll find it and see what it has to say about your compression.

Well I got lucky and the manual was the first place I looked. :mrgreen:

I agree a compression gauge will continue to creep up, but 2-3 pulls should be at least 4-6 revolutions of the engine, more than enough to give you an idea of it's condition.

For example let's say your engine should have 125 psi cranking compression & you spin it 5 revolutions.
1st= 90psi 2nd= 110 3rd= 117 4th= 120 5th=122 you're in good shape!

Now if it goes like 60-76-85-93-100 not so good. It will be hard to start (if it will start) & perform poorly.
Keep in mind I haven't had any coffee yet & I just pulled these numbers outta the air as an example, they don't pertain to anybody's engine. Results can vary due to the type of gauge being used, hose length, actual trapped volume in your cylinder due to port design etc.

If you get a reading of 80 psi dry then with oil it jumps to 130 you more than likely have a problem with ring seal due to worn/out of round cylinders, scratches, worn rings, etc.

With my stuff I'm all about the first few hits on the gauge as it's pretty much all I need to keep track of my engine.
The 3 cylinder 2stroke in my avatar....184-186 across the board with 5 revolutions. 1st= 133 2nd= 152.

Enough of this....I need coffee :lol:
Cheers, Gary

Happy New Year guys. I appreciate the detailed info. After all the homework you guys did, it looks like I have homework to do. Now is a good time to get it done before spring. I'll let you know how it works out. Thanks again to all of you!