Help with compression test

[AtlDawg]

I did a compression test on a 1985 Johnson 140 motor that did not have controls hooked up so it was cold and throttle at idle position. I got an even 145 on all cylinders. Is this reliable? Should I go back and do the test again to know what shape this motor is in?

 

[OBJ]

Re: Help with compression test

WOW!!! 145 on all tubes! Wish mine would do that. If the cylinders were in bad shape it would show cold or hot. Normally, I like to take compression with the cylinders warmed up. You may see a little difference in your readings but I would say your cylinders are in great shape. Keep them like that with regular decarbs and some TLC.

 

[AtlDawg]

Re: Help with compression test

What about the throttle not being at wide open? Could there still be leaks associated eith the intake ports? Maybe the numbers would be lower?

 

[OBJ]

Re: Help with compression test

I have tried compression readings with the throttles open and closed and it really don't make a difference from what I can see. You have to remember there are "calibrated" holes in the throttle plate to let air in with the plates closed.

 

[Paul Moir]

Re: Help with compression test

Since the exhaust ports are open at the bottom of the stroke, and air can get into them from there, there's usually little or no difference in measured compression when the throttle is closed vs when the throttle is wide open.<br />You can't read all the information you get from a compression test on a four-stroke into a two-stroke engine. All you get from it is a basic impression of what kind of condition the cylinder walls and head gasket (if so equiped) are in.<br /><br />EDIT: Post collision: I don't mean to look like I'm stepping on OBJ's toes!

 

[R.Johnson]

Re: Help with compression test

I don't know as I have ever seen two compression guage's in agreement, All you can really look for is how the reading's compare between cylinder's. Not just the guage itself, but the valve stem in the guage can effect reading's.

 

[wilde1j]

Re: Help with compression test

Just sold a boat with the same motor (I bought new in '85). Also tested ~ 150 psi all around ... must be typical for that motor? Your motor is fine!

 

[AtlDawg]

Re: Help with compression test

thanks for the info. This place is a great help and I hope I can return the favor one day. If I can just figure out if the transom mounts and T/T and swivel brackets from a 87 90 Johnson will fit my motor, I'll have the cosmetics done and be ready to go.