tgersx

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

Michael Garibay

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

kennyo

Man, I never get that lucky.

OldSchool

Aint that the truth!!!!!!

1BIGJIM

iTrader: (1)

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.

Edward R. Cozzi

No evidence of moisture in #7?

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

Back4More

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.

tgersx

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!

Griff

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.

Michael Garibay

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