Compression question - 50hp 2 stroke

[BEARCATJER]

I am currently looking at a used boat with a 2002 50hp Mercury 2 stroke engine. The private owner says the compression test shows each cylinder at 150 PSI which he claims is excellent. Maybe it is ...i dont know anything about compresion but that it does indicate the key "condition" of the engine "block", cylinders etc...right? I realize there is a lot more to an outboard than just the compression,but if that measurement is "poor" then i dont think i need to even look at anything else, righ?

Thanks for any helpful feedback etc . Merry Christmas!

 

[NetDoc]

Re: Compression question - 50hp 2 stroke

Bond-O was the first one with the completely correct answer. It's the ratio between the volume when the piston is down and when the piston is up.

High octane gas actually burns slower than low octane, as it is a more articulated (branched) molecule. The higher the compression, the slower the fuel needs to burn, and so you need higher octane. Ping, pre-ignition, is usually caused by too lean a mix, too advanced timing, the wrong octane or a localized hot spot (carbon buildup). Of course, it could be a combination of any or all of them.

When doing a compression check, it's best to do it with the engine warm. Typically, I crack the spark plugs loose, re-tighten and run the engine for a few seconds to get rid of any buildup that might be around spark plug threads. Air cooled aluminum heads are treated with a bit more care. Then I look for three things. The first compression, how long it takes for each cylinder to get to full compression and the difference between all of them. 10% is acceptable on an older engine. Much more than that, and it's time to diagnose further (like with a leak down test). Exceptionally HIGH readings across the board indicate a modified engine or oil consumption issues. Examining the plugs will help you here.

Compression checks will miss a number of problems on an engine, such as worn valve guides, timing components as well as others. It should be used as a PART of a comprehensive analysis and not as the end all determination of engine health.

 

[NetDoc]

Re: Compression question - 50hp 2 stroke

I realize there is a lot more to an outboard than just the compression,but if that measurement is "poor" then i dont think i need to even look at anything else, righ?

This is a true statement. If the compression is faulty then you don't need to look any further unless you want to repair the engine.

 

[ronsealdeath]

Re: Compression question - 50hp 2 stroke

Don't worry about the actual figure too much. Just check if all cylinder match within a degree of tolerance, say 5%, of each other. In my opinion a compression check is a little bit like a scientific experiment; you can't have any variables, to be able to compare two identical experiments. Therefore if you use different gauges, different motors, different peopl doing the test, different temperatures, different battery conditions etc... etc.... then you will get variable results.
So, forget about the actual figure and just rely on the variability between cylinders. If they are all similar, it hasn't got an issue related to that. But then again, there is a 100 things that can be wrong on a motor so just tick this one off and move on to the next thing to look out for!
My 2 cylinder Evinrude had low compression readings of around 90psi. Some people on here may recoil in horror at that. But both measured 90psi exactly so I knew everything was fine. The motor ran great and didn't have any problems with compression. If I had run the motor to full operating temp, put it on WOT and tested I may have got 120psi or something. but like I say above, you don't need to do this.