1977 85hp johnson 4,need info rebuild ?

[sporty98]

Hello,<br /><br />I have a 1977 johnson 85 hp outboard 4 cylinder.<br /> It seems to run just fine, But I decided to do a compression check on it and here is what I have. top right -118 psi top left -115 psi bottom right 100 psi bottom left 120.<br /><br /> I am not suppose to have more than a 5 psi differance. So I was wondering what I should do or if that is real bad and should have it rebuilt ? Was hoping who has worked on or rebuilt a few could answer those questions. Like I mentioned it runs fine and starts easy. I know that it is old and wanted to check it out. I would like to keep it awhile and use it 2 to 3 times a week during the summer. if I continue to use it likr that will it hurt it ? should I not go full speed in it any more due to finding out the compression variance ?<br /><br />Ryan<br /><br />sporty982000@yahoo.com

 

[JB]

Re: 1977 85hp johnson 4,need info rebuild ?

Howdy, Sporty. OMC says plus or minus 15#, so you are only a little out of spec. Try a decarbonizing procedure and run a new compression check. That old dinosaur proabbly has a lot of life left in her.<br /><br />Red sky at night. . .<br />JB

 

[sporty98]

Re: 1977 85hp johnson 4,need info rebuild ?

a de-carbonizing procedure ? could you mention what would be best to do ? or chemical to add ? the only way I know to get carbon off is to crape or grind off with a wire brush, buy taking off the top of the head. does not usually lower compresson some ? so would it be to try and remove some carbon from the other cylinders, not the one that has the lower compression, that way try and even it out closer ? or am I way off buy what you mentioned ? the local dealer says with in 5psi, but 90-105 compression is good. so really my compression is good, if it was just closer to the others, unless I have one bad cylinder .

 

[JB]

Re: 1977 85hp johnson 4,need info rebuild ?

Howdy, Ryan. The carbon you are interested in removing is in your ring grooves and can cause reduced compression readings. Carbon in the head and on the piston top cause imperfect combustion and imbalanced compression. <br /><br />Go to a different dealer than the one who gave you incorrect information. He has a hidden agenda. Buy a decarbonizing agent and use it as directed. It may result in closer readings, that is why I recommended it. <br /><br />Meanwhile go ahead and use it. No engine, old or new, should be operated at Wide Open Throttle for extended periods. Run it at 3/4 throttle, about 4500 - 5000rpm.<br /><br />Good luck.<br />Red sky at night. . .<br />JB