On a 1972 Mercury 800 outboard, what should the compression readings be? what about when the motor is cold?

Question

On a 1972 Mercury 800 outboard, what should the compression readings be? what about when the motor is cold?

Answer 1

Compression tests should be done with the engine cold, and the throttle wide open.  All cylinders should read 100 psi minimum, with all readings within 10% (i.e. 100-110 psi).  Sometimes low readings are caused by stuck piston rings, try dumping some Marvel Mystery oil in the cylinders to try and free the rings.

Answer 2

That old they should be with in 20 lbs of each other> any more than there are issues

Answer 3

if it's a motor you're looking at buying, I suggest narrowing your acceptable range to less than 7% difference from lowest to highest.

actual readings are less meaningful because test results vary according to the gauge you're using and who's doing the testing...even compression is a good indicator of motor's condition.  just be consistent with your methodology.

100 is a good rule of thumb, but on a 1972 model, I'd be happy with 92 or 95, maybe even 90, if they were all the same.