trying to help my buddy he, he has a miss at about 4500 rpm so we pulled the plugs everything looked good, so we did a compression test and on number 7 we are getting erratic pressure sometimes we get 120psi and 90 psi and then it will crank for a while and have no compression, all the rest of the cylinders have 145psi. no miss down low and it sounds great? can it be a valve? i dont understand how we can sometimes have compression. its a chevy 572 big block
thanks

This happened to me and it was a broken valve spring.Pull the valve cover to make sure. You may get lucky and be able to fix with little disassembly. Mike

Man, I never get that lucky.

Aint that the truth!!!!!!

Every time I have seen a broken or weak valve spring it will POP (backfire) at higher RPMS. The valve dosen't get closed in time because of the spring.

No evidence of moisture in #7?

Sounds like a valve sticking. Could be a weak or broken spring as mentioned.

I snapped a rocker stud on the exhaust side once...broke a rocker arm in half once...seized a lifter once, etc, etc, etc.
Let us know what you find.

well couldn't any cracks in the spring for the exhaust valve but i replaced it anyway and now all is fine. 150psi of compression!

Good news. It was weak, but not weak enough to be felt at lower rpms or cause a backfire. Your buddy should consider himself lucky.

Congrats on the simple repair but I would be concerned with the rest of the springs so stay alert.Mike