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compression question

W

wjcap4

Contributing Member
I have a 1977 85HP merc with 75PSI on all 4 cylinders. Is that too low or doesn't it matter so much as long as they are close to each other? Thanks
 
If it truely is 75 then that is low.

However, since it's even I would suspect it's simply your compression gauge.

I have two different ones that I use in my shop and they measure 15-20 psi different (between the two) on exactly the same cylinder.

Unless you have a high end gauge and get it calibrated regularly then it really is only good for confirming "even pressure" accross the cylinders you are testing.
 
How are you testing the compression. If you have 75Lbs on all 4 cylinders in an 85 then the piston domes should be melted beyond recognition.

Graham is right, try another guage with the igntion OFF using a jumper at the solenoid small yellow and big red wires and the throttle wide open. you may try shooting a small amount of oil in each cylinder as well. If it does check that low then I would be lookin to see if your top crankcase seal has popped out of the brg carrier.

that thing would have NO power at all I would think.

rgds,

Dave
 
Not an expert but I would get it up to running temp and check compression. If it is low then do a wet test which is as crankbearing said a little oil or I like transmission fluid in the cylinders. If compression picks up you might just have stuck rings. I would decarbon all cylinders. I like the soaking method with seafoam but I'm sure there are other opinions.
 
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