Inconsistent compression on 1 cylinder

My stock 1971 360 recently developed a bad miss. I did a compression test and all cylinders are 155 psi except #7 first it was 145 then 100. It is different each time I test it while the others stay the same. I am guessing it is a valve issue. How can I find out though?

Squirt a little oil into the cylinder through the spark plug hole and spin it over before taking the test again, do the test several times without removing the gauge if it is the screw in type
Please let us know your results

I put a little oil in and I keep getting 120-125 on that cylinder now.

Cracked ring?

Do a leak down test. Your problem will show.

Not sure being down 25 or 30 PSI would cause a "bad" miss. I would leak it. Maybe take a look under the valve cover.

A head gasket failure between 5 and 7 will cause strange things to happen.

- If it was mine I would just take it apart.

If you want more info before doing so, get it running again, confirm 7 misfire by pulling that plug wire while running to confirm there is no change.

Check intake vacuum and likely observe rapidly bouncing needle due to valve sealing issue. If no broken valve springs, pull the heads. JMO

That would be the next step.

But, I would still leak it and try to determine where it's going so I have an idea of what to expect to find. Even if you just use an adapter and pump some air into it.

I agree with a leak down test. a loose valve guide will act like you are describing. As you crank the engine for the compression test the valve is jumping around on the seat. The other issue might be since it is a 1971 vintage engine is valve seat recession from not having leaded gasoline. Not sure if you use an additive or not.

First thing I thought of was a bad lifter.I'm not saying that all the other suggestions.are not good, a leak would be good/easy. After that I would run it with the valve cover off and observe those valves carefully looking for something different about those ones, then take my thumb and push back/down on the rockers at the pushrod seat. Feel a couple good ones then the ones in suspicion. When things are right it will feel smooth a bad one will be harsh or feel like a smack or shock.

After I put a little oil in the cylinder yesterday it held at 145 psi every time. Same today. I cleaned the carbon build up off the sparkplug and fired it up. It runs good again. Maybe some carbon wedged a valve open or something.

Thanks for updating us, I hope your problem was as you suggested