Blown power valves are talked about like an everyday thing, in 40 years I can't recall ever holding one. Yes, I have put the back side of plenty of PV to my lips and gave 'em a "kiss", yep that one's OK. Leaking PV gaskets - yes. All the friggen time. Worse with 10% pump gas. Also, if the carb is anything "new", like last 10~15~20 years, it will have a check valve in the base plate to protect the PV from a backfire.
If you want to check the power valve, simple test - run the engine, stop. Carefully remove carb without tripping the pump shooters,wipe the base plate with a rag and set the carb on a coffee can or similar - quart sauce pan, etc. Let it sit for a while and check the base plate around the vacuum hole that leads to the PV chamber. Fuel wetness or drips into the pan = leak from something, lots of time the gasket can be a bit off center and not sealing correctly. This does not always condemn the PV, the metering block to main body gasket can cause all kinds of trouble if not sealing well.
Also, on a normal engine if you continue to open the idle mix screws, you will see a steady drop in the vacuum reading, not a bunch of jumping around. So a leaking PV will cause a low vacuum reading, relative to whatever is the normal level. Same with lean until you get to some point where its too lean to idle well.
Quick vibration of the vacuum needle screams valve seats, guide issues, etc. Yes, the carb can't "think" with all that going on. If it smooths out at increased RPM, for sure it's in the valve train. Maybe a cam lobe, that might be a stretch. As an example, on ly dual 600 t-rammed 357C with the good size hydro roller, 1200 RPM idle, 37 degrees of timing runs right at 12 in/hg steady when everything is close to right on the idle mix, plate positions and such.
Here's another good list of suspects I ran across on the web:
WHAT TO LOOK FOR,
(paraphrased from a rebuild manual)
1. A steady reading between 16 and 22 in.Hg. at idle...
This is normal for most vehicles.
Radically cammed engines have lower, less steady readings.
2. Normal range at idle, with sporadic drops below normal,
This could indicate a sticking valve.
3. Normal range at idle with needle vibration of about 2 in.Hg.,
This could indicate an ignition problem. Check plug gap, dwell (yes, electronic ignitions have dwell too), cap, rotor, and plug wires.
4. A steady reading slightly higher than normal.
This can be caused by a dirty air filter, or overly advanced ignition timing.
5. A steady reading 3 to 12 in.Hg. lower than normal.
This could indicate one or more of the following conditions,
Intake or carburetor vacuum leak, Late ignition or cam timing, Worn piston rings.
6. Gauge needle drifts slowly over a range of 4 to 5 in.Hg. at idle.
This could indicate an idle mixture that is too rich or too lean.
7. Gauge needle fluctuates rapidly between 10 and 21 in.Hg. at idle.
This could occur when one or more valve springs are weak or broken.
8. Gauge reads normal at idle, but drops slowly as engine speed is increased to 2,500 RPM.
This could indicate a restricted exhaust system.
9. Gauge reads below normal and fluctuates rapidly over a range of about 3 in.Hg. at idle, then the needle becomes steady as engine speed is increased.
Worn intake guides usually cause this reading.