La318 cylinder head question

You may be thinking in terms of just one problem; whereas I'm thinking in terms of any combination of problems.

In my experience, with a 318, there is easily room in the P/V clearance for the cam-chain to jump several teeth, and, the 318engine will keep running with retarded cam-timing. Obviously the cylinder pressure changes and the engine loses idle-rpm, as well as power, the more teeth it has jumped.
I've never had a 318 @120 psi
All my smogger 318s have run 135 at least, usually 140 or a little better.


Normally;
On the exhaust stroke, with the Piston at the bottom;
the valve has already begun to open. As the piston rises the valve is coming down to meet it.
When the piston is about half way up, the valve is about full open. After that, the valve begins to close, as the piston chases after it.
IF
the valve is very late to close, maybe the piston catches up to it.
if the lifter is pumped up, or the valve sticks in the guide, this is a very real possibility.

If it's a hard hit, the valve stem bends, and it no longer seals, and you get no compression. A light tap, could produce a light loss in pressure.
I have never seen a valve in a 318 stick, in normal use. Every stuck one that I have seen, was on a long unused engine in storage. If it only stuck part way open, likely the pushrod is bent, and the running engine immediately develops a tic-tic, until the pushrod falls out of the socket. If one stuck open, there is almost always a Second one.
But if several pistons show witness marks, that almost has to point to a jumped timing chain or pumped up lifters, or perhaps, a combination.

BTW
In have, at various times in the past, installed well-used 340 timing chains and sprockets onto a 318. In two cases, I remember that in order to get the cam-timing close, I had to advance the cam one tooth. It ran fine.