Compression too high?

[tw541]

I am working on a powerhead I plan to put on a motor I have. It is a 25302A which I believe is a 1973 Evinrude 25 hp. It came in a big box of parts I bought so I know nothing about its history. I put a flywheel on it and roped it over to check compression. I got 140/146. I have never had a motor with compression that high. Of course all my motors are fairly old. The highest compression motor I have is a 1971 Johnson 25. It has 118/122. This powerhead is showing over 20 psi higher than that. I am using a Quicksilver gauge which I have for a couple of years and I think it is fairly accurate. After seeing these readings I used it to check a couple of other motors. They read very close to what I had seen when I had checked them before.

Is this compression too high? Have other people seen these kind of compression numbers on these old Evinrudes. The head has obviously been changed at some point.

[crosbyman]

folks usually worry about low compression … if it works don’t fix it

Joining AOMCI has priviledges 🙂

[frankr]

Mixing 18-20-25hp pistons and heads will create strange compression numbers.

[billw]

It may have been loaded with oil in the cylinders to store it.

Long live American manufacturing!

[tw541]

Thanks for the reply’s. I checked both cylinders with a borescope and they look clean. No excess carbon buildup on the pistons and no visible oil. The compression is this high with dry cylinders. The 1973 18 and 20 hp motors used the same head, but the 25 is different, according to MarineEngine.com. The head has definitely been replaced but I do not know if it is the correct one. It is possible it is a 18 or 20 head. I don’t know what the difference is. I checked with my old compression tester and got similar readings.

• This reply was modified 4 years, 4 months ago by tw541.
• This reply was modified 4 years, 4 months ago by tw541.

[billw]

The 25 has a higher compression head than the 18/20s. If it has lived in salt water, you are lucky it has any compression at all. The cylinders usually develop separation between the aluminum and the steel liner and often will leak water into the combustion area. I bet if you get it running and recheck the compression, the numbers will be down a bit but still in the 125 range.

Long live American manufacturing!

[Author]

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