Compression question

[Greg O.]

I recently bought a beautiful 1957 10hp Johnson. It was late in October, so I didn't have much time to run it much before winter set in. I was checking the compression on another motor, so I decided to check this 10hp motor. I was shocked it was only 70psi on each cylinder. The motor starts and runs very well. It idles very slow, and has good power. This motor does not seem to have many hours on it judging by the way it looks and for lack of wear on parts like the shift linkage and throttle linkage. Will a de-carburization help the compression much? Or should I re ring the motor, or just de-carb and run it? What is an acceptable compression pressure for this motor? Thanks for your imput.

 

[kbait]

Re: Compression question

Don't get too caught up on actual compression #'s.. within 10% for each cylinder is what you really want to find. You don't get much for rpm's w/recoil start, and often won't get a high psi reading. I say decarb and go..

Good luck!

 

[OptsyEagle]

Re: Compression question

You could also look at changing the head gasket before you do a decarb. It is easy to do, inexpensive and will allow you to clean the carbon from the pistons manually, before the decarb. It also allows you to flatten the cylinder head and re-torque the head bolts as well.

Then do a decarb and see if it helps. Even if it doesn't, I doubt I would re-ring it with the numbers you got.

 

[JB]

Re: Compression question

70psi is acceptable on a rope start outboard, particularly if both cylinders are within 10%.

 

[Greg O.]

Re: Compression question

70psi is acceptable on a rope start outboard, particularly if both cylinders are within 10%.

The cylinders are both exactly the same, 70psi.

 

[Crosbyman]

Re: Compression question

if it works don't fix it ;-)

 

[Bosunsmate]

Re: Compression question

sounds good, id do a decarb anyhow to clean it up inside

 

[boobie]

Re: Compression question

Agree+1...........

 

[Greg O.]

Re: Compression question

Thanks for all the input. The motor runs great, so it surprised me when I was only getting readings of 70psi. I will de-carbonize it, and a seal kit and impeller, and tune up, and call it good. I love these old OMC motors.