[crackedglass]

I have two Mercury 40 HP twin cylinder engines. One is a 1971, the other a 1985. The 1971 gives me a compression reading of 145/147 on both cylinders, the 1985 shows only 122/122? Both motors are well maintained freshwater run motors, both seam to run great. The reason for my concern is that all of my other Mercury motors show in the 140 to 150 psi compression range on all cylinders. But this recently acquired motor shows less.

I'm just curious if they lowered the compression or could this motor be just weak? Running wise it gives me no reason for concern, but since I was checking the plugs and winterizing it, I figured a compression check was in order.

Do you think that this motor may be 'well worn' or is it normal compression findings for such a motor?

I realize that all that realy matters is that the cylinders are all within 5 or 10% of each other but what constitutes to much wear or noticable wear?

[northernmerc]

That compression sounds OK. I have a '78 Merc 40 hp with about 130 psi compression on all cylinders and a Johnson 30 that is at about 120 psi. One hears about motors with compression below 100, and the owners don't seem concerned.

[JB]

Both engines are just fine, CG.

The owner appears to be suffering from psi paranoia.