Buying this used boat ? Compression ? 1995 Johnson 150

[Will S.]

Checking out a boat I want to buy. My drive test ran great, (smoked a bit when cold start, but cleared up pretty fast), shifted good, quiet and smooth. The motor itself was very quiet for a 2 stroke. Hole shot was good and good top end speed for the boat. The shop checked Motor over and said everything looked fine. However..... compression was all with-in 3 psi. But the readings In all 6 cylinders were between 72 & 75 psi. If I had not had it checked, I would have thought it was a great deal. I can not find anything good about these results, but since they are all tight in psi, makes me wonder what this motors compression SHOULD be.. and thoughts of how to process these results? Thanks.

[aarchdale@coresleep.com]

I dont know what it should be but i do know the gauge can make a difference. I did a check on mine once and they were all around 95.  I went and rented a compression tester form orielly and tested the exact same way and all of them went up to 120.

[Will S.]

I believe MONDAY, I will request the “full time “ mechanic recheck with a different gauge to verify these numbers are correct.

[fishinwrench]

Yeah I'd question the guage that was used.  Anything 90+psi is fine as long as they are all within 15% of each other.  

The thing with compression tests is that PEAK compression isn't necessarily what you should be looking for.  On each cylinder give it the same number of compression strokes, and record THAT number.   Even a cylinder with sloppy rings will pump the compression up....given enough strokes.  Since 15% is the magic number I divide that by 3 and give each cylinder 5 full compression strokes and record THAT number.   By doing that you are not only testing compression but you are also testing leak-down at the same time.  

By cranking it over until the guage peaks out you really aren't learning anything about the overall health of the powerhead. 

Since you have a easy start, smooth idle, strong hole shot, and a good top end..... I'd say you're fine.   What more can you ask from an outboard?

[MrGiggles]

Gauge was probably leaking, or had way too long of a hose. If the Schrader valve is not placed close to the end of the hose, the volume of the hose adds to the combustion chamber volume, effectively lowering the compression ratio.

It's why you get low readings when using an automotive gauge on a chainsaw.

If it truly had compression in that range, I imagine there would be other symptoms.

[Will S.]

update: I have decided to pass on this sale.

[fishinwrench]

Well please link us to it.  I may buy it.  😊

[Will S.]

20 minutes ago, fishinwrench said:

Well please link us to it.  I may buy it.  😊

He took the listing down.