Mercury 500 Thunderbolt compression readings

[spunkymonky]

I'm replacing the powerhead on my engine and have found one that seems in good shape and looks pretty clean.

Compression readings from top to bottom are 130 145 142 130.
I realize that's not within the 10% recommended but still decent numbers I think, as my last one was 100psi +/- 5% across the board.
What might cause the top and bottom cylinders to be that much lower?
Should I just throw it on as is or try to diagnose the issue prior to bolting it up and running it?

1972 Mercury 50Hp / 500 Thunderbolt

 

[tpenfield]

The numbers are barely out of a 10% range, so not a huge deal. Did you run the compression test with the throttle wide open? If not that could account for the difference, otherwise hard to tell.

I'd go ahead and use the engine.