Compression on evinrude 40hp

[Jhyatt1020]

I have evinrude 40 hp 2 stroke compression in 130 on top and 110 on bottom...it runs good in yard when you put it in water it will start and cut off will not idle..can someone please help me..could it be because compression is the same

 

[BruceK]

Difficult to say as it could be any number of reasons but that engine is not healthy with those compression readings. There should be a max of 10% difference between cylinders. I'd certainly rectify that first before trying to tune to compensate.

 

[gordmac]

Compression hot or cold? Throttle open or closed?
Low numbers for four stroke but I suspect two stroke has lower numbers anyway, not my area.
I don't think compression is your problem with those symptoms, I suspect carbs need cleaning. Cheaper to try that first.

 

[BruceK]

agreed but that percentage difference in compression is always going to lead to a rough idle and the tendency then is to try and tune it out. That in itself leads to further issues.

 

[gordmac]

Indeed but I would try a few cheaper things before sticking a set of rings in it. I am not too knowledgeable on two strokes but on car engines I have seen compression variation on cold engines with shut throttles improve when checked hot on open throttles.

 

[BruceK]

Absolutely. If the compression can be done warm the compression figures should rise slightly, but not too much. If they do there is a problem. If tested wet with a bit of 2 stroke oil down the plughole first and the pressure increases again then he will know for sure it's rings. This should be complemented with a leakdown test as well. But even a cold test the percentage difference should not exceed 10%. That's a problem, not that he is making only 110 on the bottom cylinder which if anything should be the wetter of the two on a cold test. 110 is a bit low but the engine should still run on it. To me it sounds as if the engine has soft seized at some point.