what should the compression numbers be?

[jamesbalog]

going to look at a 1981 merc 200 tonight.... was wondering if someone could help me out in letting me know what the compression numbers should be on that motor?

thanks

[smokeonthewater]

Biggest thing is they should all be within 10% of each other.

Test with all plugs out and at wot

I'm gonna say look for 120 psi or more +/- 10%

[spareparts]

Originally Posted by smokeonthewater

Biggest thing is they should all be within 10% of each other.

Test with all plugs out and at wot

I'm gonna say look for 120 psi or more +/- 10%

what he said, probably more like 115. Don't get bent out of shape if its lower or higher than that. Most compression gauges aren't calibrated,I tell people, I've got a buying(reads low) and selling(reads high) gauge, which one do you want me to use. Look for them to be close to the same number, rather than worrying about the number itself

[smokeonthewater]

fwiw I just bought an engine for my truck ... compression ranged from 350 to 380... was a cold engine that hasn't run in 2 years.... New on this one is about 450 but since it was a cold engine and had set so long I'm satisfied... those numbers will come up with some run time....

The same applies to boat motors... You won't usually get the best numbers on a cold engine especially if it has set long enough to get even the slightest surface rust in the cylinders.... If you possibly can get it up to operating temp before testing..... It only burns ur fingers a little lol