30 psi compression in 1cyl what to do?

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parrisw

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Jun 8, 2009
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Well the boat I just got, the guy told me 1 cyl was down 10psi, that's not too bad. But since it doesn't like to idle too well, I decided to check compression, which I should of done before buying. Its a 1980 Johnson 115 its the right side top cylinder is about 30psi, all others are at 120-125. What would be the first step? Pull the head?

Thanks
 
Bifflefan

Bifflefan

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Re: 30 psi compression in 1cyl what to do?

Probably a broken ring, you hope.
If you have no other way to see the cylinder walls, then you can pull the head and look at the walls for scratches. then grab the piston and give it a good rattle. if it moves more then just a bit then you will nee a piston also.
 
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parrisw

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Re: 30 psi compression in 1cyl what to do?

bifflefan said:
Probably a broken ring, you hope.
If you have no other way to see the cylinder walls, then you can pull the head and look at the walls for scratches. then grab the piston and give it a good rattle. if it moves more then just a bit then you will nee a piston also.
Click to expand...

Ok, thanks. Can this be done with the powerhead in situ?? Or does it have to come out to get a piston out?
 
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wilde1j

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Re: 30 psi compression in 1cyl what to do?

The power head must be removed and torn down. If you're planning to do it yourself, get an OEM shop manual first. Also, you need to find out why the two holes went down, or it will surely happen again if you don't.

I would pull the head first and try to determine the extent of the damage.
 
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parrisw

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Re: 30 psi compression in 1cyl what to do?

wilde1j said:
The power head must be removed and torn down. If you're planning to do it yourself, get an OEM shop manual first. Also, you need to find out why the two holes went down, or it will surely happen again if you don't.

I would pull the head first and try to determine the extent of the damage.
Click to expand...

Ya, I'll be doing it myself. Yes I want a shop manual. Cant find a oem one yet, maybe aftermarket one. Its only 1 hole low. How do I find out what went wrong? Say it is just a broke ring? Could it just be that? Maybe it overheated, and only blew the one cylinder.
 
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wilde1j

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Re: 30 psi compression in 1cyl what to do?

Or ran lean in one hole due to a variety of factors (crap in carb, air leak, etc.). Or had a load of carbon in the exhaust, etc. Lot's of possibilities. OEM book is the only one to get if you're doing serious work.
 
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parrisw

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Re: 30 psi compression in 1cyl what to do?

wilde1j said:
Or ran lean in one hole due to a variety of factors (crap in carb, air leak, etc.). Or had a load of carbon in the exhaust, etc. Lot's of possibilities. OEM book is the only one to get if you're doing serious work.
Click to expand...

Ok, thanks for the tips. Where else to look for OEM book. I'd much rather have that.
 
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Daviet

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Re: 30 psi compression in 1cyl what to do?

Google ken cook, he has OEM manuals.
 
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parrisw

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Re: 30 psi compression in 1cyl what to do?

Daviet said:
Google ken cook, he has OEM manuals.
Click to expand...

I think I checked there, and no OEM manuals for my model. I'll look again.

Also, can I take the powerhead off without taking the whole outboard off the boat?
 
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Daviet

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Re: 30 psi compression in 1cyl what to do?

Yes, you can remove powerhead without removing motor from boat.
 
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parrisw

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Re: 30 psi compression in 1cyl what to do?

Daviet said:
Yes, you can remove powerhead without removing motor from boat.
Click to expand...

Good, thought so.

I'd rather just buy a newer motor, but $$$$$ $4-5k for a good used motor, then have to change controls, and steering as well???
 
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ezeke

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Re: 30 psi compression in 1cyl what to do?

Could be a blown head gasket; I would take the cylinder head off and examine before anything else.
 
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parrisw

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Re: 30 psi compression in 1cyl what to do?

ezeke said:
Could be a blown head gasket; I would take the cylinder head off and examine before anything else.
Click to expand...

Yes, that's what I was intending to do, to hopefully see what the problem is.
 
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Daviet

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Re: 30 psi compression in 1cyl what to do?

The one on Ebay looks good as long as it lists your model number on the front.
 
kenmyfam

kenmyfam

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Re: 30 psi compression in 1cyl what to do?

Did you check the head gasket yet ??
 
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