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trying to evaluate my 1960 Johnson 75hp.

Posted: Sat Jan 26, 2013 1:37 pm
by Retroman
Would you mind taking a look? Any advice is greatly appreciated.

http://www.youtube.com/watch?v=AZUJKqKMoUw

Re: trying to evaluate my 1960 Johnson 75hp.

Posted: Sat Jan 26, 2013 2:27 pm
by vupilot
You say youre still getting good compression on your compression guage? Checking warm will usually error on the high side, a cold check sometimes will be lower. Thats a pretty good amount of piston damage, I've seen similar damage from an engine that was revved really high straight after a cold start or it could have been operated normally but overly lean to cause damage like that. Hard to see the damage of the cylinder in the video but from the pictures the part I can see doesnt look all that bad.

Re: trying to evaluate my 1960 Johnson 75hp.

Posted: Sat Jan 26, 2013 4:44 pm
by Moeregaard
I see some carbon buildup, but the bore looks pretty good.

-Mark Shipley

Re: trying to evaluate my 1960 Johnson 75hp.

Posted: Sat Jan 26, 2013 5:35 pm
by Roberta
The most important thing is the seal between the rings and the cylinder walls. Scuffing on the pistons could be caused by them rocking from predetonation from the carbon buildup and/or running lean due to the carb being loose or possibly carbon deposits breaking off and getting between the cylinders and pistons. How does the compression you got compare to what the MFG says you should have and how does it compare to the other cylinders? should be within 10-15% of each other. For sure, clean up the carbon on the upper cylinder walls and the top of the pistons.

Roberta

Re: trying to evaluate my 1960 Johnson 75hp.

Posted: Sun Jan 27, 2013 2:09 am
by Retroman
Thanks Guys,
The compression is within 5lbs of each other, at about 80 pounds per, done on a cold motor. Im gonna put it back together, run it and hope I get a season out of it. I wont invest much money, as the motor is damaged.
Thanks again
Jim

Re: trying to evaluate my 1960 Johnson 75hp.

Posted: Sun Jan 27, 2013 11:55 am
by Retroman
Another silly OMC question. It seems odd, only one cylinder was damaged. The carb may have been loose, as the bolts are stripped, and it was pointed out to me this could have caused a lean condition in the one and only damaged cylinder. I was curious if the loose and frozen bolt in the pic, could have let exhaust gases seep into the cooling system and affected only the top cylinder in this photo??

Re: trying to evaluate my 1960 Johnson 75hp.

Posted: Thu Feb 07, 2013 9:51 pm
by Caber-Feidh
Not that bad It does not look like a typical lean run. Don't worry about predetonation, you couldn't make those old low-compression engines knock on nitroglycerine.

Scuffing like that is typical foreign object/material. I have seen excessively carbonized engines scuff very much like that. The scuffing looks like it runs all the way above the top of the upper ring apex. It is likely from chunks of carbon trapped between the piston and the cyl.The polising around the ports shows the bore is out of round, the area around the ports is a weak spot, and will deform with age, use, abuse. It's not significantly bad, just an observation. If you really wanted, you could drop a hone down it, and re-ring that piston, but IMNHO it's not that far gone.

The old dino-oils were not even close to a modern synthetic, about every non-chorme/nikasil jug you see on a 50 year old 2-smoker will look like that from oil coking. Don't worry about it. Knock off the loose carbon, don't let it get into the case. go 5 points richer on the mix to deal with some possible roughness, and use a decent synthetic instead of that bug-fogger fluid from the last century, and if you have fixed any possible vacuum leaks that old wheezer will go another 10 years... maybe at 50HP, but it will keep going.