Low compression?

[huntin dawg]

Hey all,
I have a 4.5 Johnson that siezed up. After it cooled it freed up no problem. However, I found water and mud in the cylinders so I replaced the head gasket and side plate gasket. The pistons and cylinders looked good except for a couple spots of mild scoring from the overheating/siezing episode. So I checked the rings. They are all nice and free except for one that acts a little sticky. After putting her back together I did a compression check on it cold. I'm getting around 60 PSI per cylinder. Is that normal on a cold engine? I hope she'll start so I can consider my work a success. Any thought?

Thanks.

 

[stingertrey]

Re: Low compression?

It seems to be a lil low but as long as they are within 10% of each other it should function and its cold so it may go up a lil.. and you will always get a lower compression reading on a pull start engine.. rarely do ppl get 100 psi on pull start engines from what I read.. Hopefully you didnt do to much damage.. If you cant hold your hand on the head for about 3 sec while its runnin then its definitely overheating

 

[Rudi2]

Re: Low compression?

My old Johnson book states compression should not be less than 100 psi. I'd do a wet test, squirt some oil in cylinder and run comp test again. If it jumps up 10 psi or more you're probably needing rings, at least.

 

[R.Johnson]

Re: Low compression?

You found water, and mud in the clyinder. I don't know how mud could get in there. Sound's like you could have been looking at oil mixed with ground aluminum.

 

[kbait]

Re: Low compression?

Run it. Those 'lil motors are quite forgiving, mostly because of the 2 cyl. design. It'll likely run great. Do a decarb too.

 

[huntin dawg]

Re: Low compression?

You found water, and mud in the clyinder. I don't know how mud could get in there. Sound's like you could have been looking at oil mixed with ground aluminum.

I should clarify. When it freed up I was trying to get it started. It was on my boat in a very silty Alaskan creek. The mud was in the water pumping through it. Alaskan creeks and rivers can be dark gray from the incredible amount of silt in them.

Thanks for the replies.

 

[jbjennings]

Re: Low compression?

Here's my take on it......
I've read where several guys had 60psi on their engines and they ran fine. I personally think 60psi is terrible compression. But if it runs fine, who cares.
I've never seen a motor with 60psi on both cylinders except a mercury 3.9 and I NEVER got it cranked. The only OMC that had 60psi on one cylinder, had 85 on the other and the 60psi cylinder had a totally smashed wrist pin on it.
I also have NO problem getting 115 and 125 psi readings on my 15, 18, and 25hp outboards from the 50's with the rope starter. If I recall, the electric start just gives maybe 3-5psi higher than the pull starter, if any.
Compression gauges are known to be unreliable unless they're checked against a known compression engine or other method. You may just have higher compression than you think. See how she runs, that's what really counts.
Good luck,
JBJ

 

[JB]

Re: Low compression?

Small, rope start JohnnyRudes often compress around 50-60psi when in fine health. If both are 60psi it is good.

I never, repeat never, saw two cylinders fail by the same amount at the same time and I never, repeat never, saw an outboard with worn out rings.