1962 Johnson 75 hp low compression

[daveydb]

I just tested the compression on my 4 cyl johnson and they are all registering around 15 or less. I saw in the manual something about some type of "reed" in the pistons and was wondering if this could be part of the problem. If so, what is it and would it be more to blame than the piston rings? I assume it would be easier to replace than the rings but I am not sure. I assume those are the majority of the things that can cause low compression. Thanks for the help.

 

[F_R]

Re: 1962 Johnson 75 hp low compression

15psi is terribly low. No, the reed valves don't cause that. To have only 15 on all 4 cylinders is unusual. Has that motor been run hot? If the paint on the cylinders is burned or discolored, it has been. If you are confident that your compression gage is ok and you are cranking it till you get the highest reading, and the head gaskets aren't blown, then the motor is needing major rebuilding.

The thing is, it could not have run long enough to get the compression that low on all 4 cylinders unless it was overheated. There is something you are not telling us here.

 

[tashasdaddy]

Re: 1962 Johnson 75 hp low compression

i think i would try again, turn the motor over with the starter, until the pressure quits going up.

 

[daveydb]

Re: 1962 Johnson 75 hp low compression

I bought this from a guy last year that said he didn't have any problems with it. I don't know if he ran it hot or not. I haven't run it enough to cause it to overheat, but I guess it could have happened before. Do the reeds contribute to the compression?

 

[tashasdaddy]

Re: 1962 Johnson 75 hp low compression

no. try compression again. hard to believe all 4 are that low. are you using screw in gauge.

 

[daveydb]

Re: 1962 Johnson 75 hp low compression

You were right!! I finally got my Dad to get a screw in one and retested the compression and it was up around 80 on all cylinders. He then took it out and it ran beautifully after doing a little carb cleaning.

One new problem though. He can't get it to go in reverse. Forward works great but it just won't engage the reverse gear. We had taken the lower unit off a little while ago and I was wondering if moving the wires could have made this happen. Thanks

Dave

 

[F_R]

Re: 1962 Johnson 75 hp low compression

Wire problems can make it not shift if you have the Electramatic Shift model. Be careful messing around with the wires, you can bust a drive shaft or clutch if you make it go into both gears at once.