1978 Johnson 35hp Compression

[shooter5636]

I've been back to searching for a motor after the '79 I checked out came up a dud.

I've been doing a lot of reading on outboard compression, but I can't seem to find what I'm looking for. I did a compression test on the motor cold and both cylinders turned 82psi on the dot. Motor started up super easy, idled great, and sounded healthy. The service manual for the motor stated that a specific compression range wasn't important. What is important is that both cylinders be within 5psi of each other. They are spot on.


If it were you, would 82psi of compression on each cylinder raise alarm?

 

[Joe Reeves]

If you ever find a compression ratio quoted by the manufacturer of any outboard, let us know. Personally I was in the OMC trade from 1960 to 1991 and have never come across it. The compression is generally burnt into our minds when the engine is brand new and stays there throughout its existence for reference.

Your quoted 1978 35hp Johnson model's compression is usually up in the 100+ psi area... BUT... compression depends on whether a cylinder is cold or warm and lubricated or dry, even if the cylinder, piston, and rings are in perfect condition. A important factor is also the speed of which the engine is cranked over.

As you state, the compression on all cylinders should be fairly close to each other. The engine you checked out has a perfect cylinder to cylinder comparison. I'd suggest, if the compression test was on a cold engine that hasn't been run in some time, to try squirting a bit of premix into the cylinders to see what psi compression increase might exist.

 

[gm280]

Along with the good advice provided above, I would also make certain your compression gauge was good. I say that because rarely do you get two "exact" readings when testing compression. Most of the time, one will be just a little different. Not by any large number, but not exact eithe, even a needle width would be better knowing the meter wasn't stuck at that range when testing. So I would verify the gauge and go from there. JMHO

 

[shooter5636]

Thank you guys. The guy selling the motor did the test with his gauge. I'll bring mine and do it again.

If the results hold true, I'm wondering if the rings are stuck. I just can't get over how well it started and ran.

 

[boobie]

If it runs as good as you say, I'd take it and run it.

 

[shooter5636]

I checked out the motor 3 separate times, and each time it kicked over as soon as you hit the starter on a cold engine. Ended up bringing it home. I used a bore scope to check out the cylinders and pistons. There's a good amount of carbon which I'm thinking means stuck rings. There is what looks like light scoring on the cylinder wall, but not bad enough for me to be thinking about a rebuild. Ill decarb and keep an eye on the cylinder walls, but overall I'm happy. I got a killer price on it.

 

[gm280]

I checked out the motor 3 separate times, and each time it kicked over as soon as you hit the starter on a cold engine. Ended up bringing it home. I used a bore scope to check out the cylinders and pistons. There's a good amount of carbon which I'm thinking means stuck rings. There is what looks like light scoring on the cylinder wall, but not bad enough for me to be thinking about a rebuild. Ill decarb and keep an eye on the cylinder walls, but overall I'm happy. I got a killer price on it.

If you made a good deal on it and it runs great as you say, why worry about any type of rebuild. I say run it and enjoy boating with it. :thumb:.

 

[Crosbyman]

wouldn't worry to much about he rings at this point why not just get some oil in those cyl and let it soak a few weeks to loosen it up. PB blaster may be even better

​then run a good decarb product in it I hear Mercury Engine tuner is pretty good or Seafoam.

IF worried you can always just open the big side inspection cover to access the rings for direct application a PB Blaster like product to both sets of rings

just mho

 

[shooter5636]

By stuck rings, I was referring to the carbon build up. It's currently soaking in sea foam, and I'm going to follow the rest of the decarb procedure. The real test with be thanksgiving morning when I hit the river to chase some ducks and geese. I'm pretty pumped. This is a big jump over the 15hp I had on the boat.

 

[Crosbyman]

bring a kicker just in case... kind of cold on the water rowing against the wind picking up ducks... gave up on ducks myself ..... years ago

 

[shooter5636]

That's the plan! The 15 is coming with just in case.

 

[Crosbyman]

don't flip over heading in the wind....

​believe it or not a buddy and I were heading out to our duck blind, opening morning at 4 a.m. one year, when I heard a soft HELP...!!! Help!!! (Fraser point.... lake ST-Francis Quebec)

​one hunter was hanging for dear life on his flipped over 14 footer decoys, motor gun all gone . The strong head wind and heavy back load flipped him over

​we picked him up ..took him to shore dripping and crying, left him with my buddy & I went back out for his empty "rented boat" boat When I got back he had already jumped in his car and disappeared from sight.

​there is still a motor and shotgun down there somewhere !

 

[shooter5636]

don't flip over heading in the wind....

​believe it or not a buddy and I were heading out to our duck blind, opening morning at 4 a.m. one year, when I heard a soft HELP...!!! Help!!! (Fraser point.... lake ST-Francis Quebec)

​one hunter was hanging for dear life on his flipped over 14 footer decoys, motor gun all gone . The strong head wind and heavy back load flipped him over

​we picked him up ..took him to shore dripping and crying, left him with my buddy & I went back out for his empty "rented boat" boat When I got back he had already jumped in his car and disappeared from sight.

​there is still a motor and shotgun down there somewhere !

I think it's a safe bet you saved a life that day. I see a lot of very stupid things on the river I hunt. No duck is worth your life.

I'm going to secure the 15 up towards to bow with my other gear to even out the weight. I actually skipped hunting today because of the +40mph winds we have.

 

[boobie]

The only time I ever saw OMC quote any compression # 's was in the 1968 16 hp snowmobile manual. They said you had to have a minimum of 85 psi for the motor to run properly. Wish I could find an old manual.

 

[Joe Reeves]

The only time I ever saw OMC quote any compression # 's was in the 1968 16 hp snowmobile manual. They said you had to have a minimum of 85 psi for the motor to run properly. Wish I could find an old manual.

I believe I have a 1966 OMC manual that covers more than one model (only multiple model manual I've ever seen from OMC). I'll look into that manual ASAP.

 

[boobie]

Joe, keep me posted with what you find but this was 1968.

 

[Joe Reeves]

Joe, keep me posted with what you find but this was 1968.

Boobie..... I was aware of the 1968 factor but this brought to mind that old multiple model manual I had. I found it just exactly where it should have been in my file cabinet. It takes in the models from 3hp to 90hp and is quite thorough with a minimum of confusion.

Johnson Outboard Service Manual - Comprehensive All Models, Part No 312468, covering the 1965 & 1966 models.

The coverage is really good... BUT... the only mention of "Compression" pertains to what might interfere with what the compression should be... BUT... nowhere in this manual does it actually even hint as to what the compression should be. You would think they would give a starting point.

What if someone was getting canned and on their last day they accidentally left some piston rings out etc... and the engine wasn't test run at the factory. Yeah, I know they say they run them all... BUT... one time the rigging crew (big dealership in Tampa) called me out to their area because they couldn't get a brand new, just installed three (3) cylinder model to run properly. In removing the cylinder head, I found one of the pistons installed (at the factory) upside down! Impossible that they ran that one!