2005 F40tlrd four stroke compression

[mikerj11]

What would be good/normal compression for a used 2005 40hp four stroke, seller says under 60 hours on outboard. I was under the impression there is know way to tell how many hours are on a carbureted outboard??? But the compression test was done, he said it was 90psi on all three, said he did the compression test on all cylinders at once??? All of this seems to contradict most things I've read about compression tests. any insight would be appreciated, it's on a pontoon and looks clean, but the guy at the marina said look at the propeller it's clean (like that can't be replaced, I almost laughed) as a reason that the engine is good. I own two different Yamaha outboards 115 four stroke on a blazer bay and a 25hp four stroke on a 13 whaler bought new and still love em. just do the regular maintenance and have had no problems, but this is the first used I've looked at.

[boscoe99]

The motor itself (carb'd Yamaha) can't tell you how many hours are on it but some tachometers can, if installed new when the motor was new, and if operated properly.

If someone has three compression gauges there is no reason why all three cylinders can not be tested at the same time but to me it is unusual. Might prevent the starter motor from rotating the pistons as fast as they would be rotated with only one cylinder being tested at a time, in which case the compression values might be lower than they would otherwise be.

Seeing the exact same pressure (90psi) on all three cylinders seems suspect. I would do, or have done, a test for myself. Engine hot, battery fully charged, throttle wide open, known good test gauge, each cylinder tested individually and the motor cranked until the pressure stops rising.

I would expect to see 125 psi or more in each cylinder in a motor with only 60 hours time on it.

[99yam40]

do not take someones word on anything unless it is from someone you hired to do the tests.
Plus I would like to be there when they are done to see for myself.

Have a leak down test done, not just a compression test

I thought the 4 stroke had more compression than that

[mikerj11]

Thanks, new aluminum props and the same psi on all three cylinders and no way to tell how many hours on the engine were sending red flags to the moon, appreciate the comments.

[99yam40]

a good hard run on the water loaded up at all RPM for a long enough time to make sure it performs properly should help you see how it runs