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Compression check results

2630 Views 12 Replies 5 Participants Last post by  fsae99
Hey gurus!

My 2005 30 hp Johnson 2-stroke with electric start was making a strange grinding/clanging sound at high RPM or in the mid-range under load (plowing or turning while trying to get on plane).

I did a compression test today and got an equal 90 psi in both cylinders. I was expecting to get closer to 100, but they're both equal and it doesn't seem terribly low.

My owner's manual says nothing about cylinder compression.

How do these numbers sound?

BTW, when I pulled the cowling cover off I noticed that the starter pinion was engaged. It is showing a small amount of rust in the gear teeth. If this is not disengaging after the motor starts, I'm thinking it could be the culprit.
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· devilray snob
2,409 Posts
Did you just use a cheap $20 compression tester? Those tend to not be too accurate as far as their readings go, but if both cylinders are the same you should be OK. This is what I've read doing research though, I'm not an expert.

I have a cheap compression tester I got on Amazon for like $14...on my 2002 mercury I get 110PSI on both cylinders with no % difference between the two.

I'm 'supposed' to be getting around 120-125PSI, but I attribute that to the gauge being shitty and cheap. As long as they match up it seems that they should be fine.

I could be wrong, though ;)
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