92 Johnson 150 Nitro low compression - replace rings or pistons?

D

DIAT

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1992 Johnson 150hp motor model number TJ150GLENC. Did a compression test last night the right side of the engine reads 100, 95, 90 and the left 90, 87, 85. Pulled the head off and looked at the cylinders they look good with only a couple very minor marks. We are going to hone it assuming it's within limits. My question: do I replace just the rings or do the pistons too? What the major drawbacks of doing just the rings? and does anyone have an idea what the compression on this motor should be at?
 
dwco5051

dwco5051

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Re: 92 Johnson 150 Nitro low compression - replace rings or pistons?

If the motor is in running condition the first thing I would do is decarb it and recheck the compression. Around 90-92 psi is what my 1991 175GT shows on all cylinders. Less than 100 hours since last overhaul. The variance between cylinders on your motor is a little out of tolerance and may just be due to rings being gummed up in the grooves. Worth a try and cheap compared to a tear down and rebuild.
 
jimdd810

jimdd810

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Re: 92 Johnson 150 Nitro low compression - replace rings or pistons?

I agree and the compression tester you are using might read differently. Compression test is only one diagnostic test you can do you can also do a cylinder leak down test which will give you much more information. you do need a tester and a compressor but could save you lots of bucks.
Is it running poorly what made you decide to do the compression test?
 
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DIAT

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Re: 92 Johnson 150 Nitro low compression - replace rings or pistons?

When starting from a dead stop it would die if i gave it full throttle, at low running speeds u could smell more gas then u should, and when we pulled the plugs they looked rich. We were going to clean the carbs last night but decided it would be worth testing the compression (which is when my night got a little worse) and then decided to check the cylinders. Tonight I'll take some pictures of the cylinders and post them 1 of the cylinders had a pretty large gap compared to the others and also had the light markings which is what started the ring and piston debate. Thanks for all the great tips!
 
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jonesg

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Re: 92 Johnson 150 Nitro low compression - replace rings or pistons?

DIAT said:
1992 Johnson 150hp motor model number TJ150GLENC. Did a compression test last night the right side of the engine reads 100, 95, 90 and the left 90, 87, 85. Pulled the head off and looked at the cylinders they look good with only a couple very minor marks. We are going to hone it assuming it's within limits. My question: do I replace just the rings or do the pistons too? What the major drawbacks of doing just the rings? and does anyone have an idea what the compression on this motor should be at?
Click to expand...

Comprssion is not that far out , its right on the edge of spec and well within the margin caused by excess carbon.

It should have been decarbed and retested before removing the head.
I would not do what you are planning.

Install new head gaskets, retest compression , then decarb with seafoam or whatever you have for decarbing and test the compression again, I think you'll find its good. Mine came up from 70psi to 85psi in 30 minutes without removing the cyl heads.
80-90 psi is fine for big V6's.
 
jimdd810

jimdd810

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Re: 92 Johnson 150 Nitro low compression - replace rings or pistons?

I agree then rebuild the carbs sounds like it to me. Also change your plugs
 
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