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Compression check results

2631 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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· Cert. Yamaha technician
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4,360 Posts
^ your right
Did an experiment inthe shop one day, as a bet.
One of the guys that just bought a $350 snapon tester was acting like an azz so we all compression tested the same motor, with out different guages.
There was a 20lb difference from the lowest to highest guage.
But they were all equal, and thats what matters.
If your ever unsure , always try a second tester before taking the motor apart :cool:
 

· Cert. Yamaha technician
Joined
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4,360 Posts
As far as the op's question goes:
On some motors the starter armature is supposed to stick up in the flywheel teeth but go down once started.
Go outside and start the boat up with the hood off, if the armature goes down once its running your good to go. If it stays locked in the teeth, you gots problems
 
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