I think it should provide a controlled flow all the time. Less flow at idle but still enough to produce positive ventilation and most flow under load.
The spring would regulate. If it was closed at idle you would build up pressure and force fumes/gunk out other areas like the breather on the opposite side valve cover.

Mine went bad and I started having oil around my VC breather and the center bolts in my valve covers. I then noticed fumes at idle coming from VC breather.
I tried 4 before I found one that worked with my lower vacuum.

I even tried one (made for a suburban I think) that had no spring or valve inside it had just a small vent hole. It didn't work for me.

 

I know that is what you think...but that is not how it works. But we all do what we must do....

 

Here's what I've read and learned about PCV valves over the years:

A PCV valve provides a controlled / calibrated air leak at certain engine regimes, mostly mid-throttle (cruise) in order to continually recycle/combust the "air" in the crankcase, avoid the buildup of moisture, etc.

It replaces the older style breather hose /filter assembly which merely dumped that stale "air" overboard, including any oil mist it contained. Blowby, etc. was a significant source of air pollution back in the day hence the addition of PCV valves sometime in the mid '60s. EPA?

A PCV valve is vacuum operated. It opens at PARTIAL vacuum levels. Not high vacuum levels like at idle, nor at zero vacuum level like at full throttle.
An internal spring determines the vacuum level at which it opens.

Opening at idle would have major impact on the idle speed, mixture, etc. You might even get a feedback loop, surging idle. I disagree with those who claim it should
"remain a little open at idle"

Mfr's spec'd a certain PCV valve, opening at a certain vacuum level, to go with certain carburetors, compression ratios, setups, etc. It had to be calibrated to prevent an over-lean
condition when it did open.

Many newer models are adding an oil catch-can to the PCV system, in order to trap & condense oil fumes, and drain them back into the crankcase. This is especially
true of super/turbocharged models which will otherwise tend to "suck" in a lot of oil fumes at high level of boost, LOWERING the effective octane level of the fuel, and
reducing power, supposedly. It will also reduce oil consumption.

When you've got an engine with who knows what mods, non-original carburetor, etc. you're shooting in the dark on PCV valve selection. If you feel it's opening excessively
you could put a restrictor in/on one of the hoses. If it's not opening as much, as in OP's case.... try some other ones?

There is a vintage PCV valve tester I see on Ebay now and then that SHOWS the pressure at which it opens. You could do similarly with a vaccum pump, gauge, etc. till
you find the one with the opening pressure you want.

In general you want the PCV system to maintain a SLIGHT vacuum in the crankcase. Suck out more "air" than is blowing past the rings. Why? Because if you allow the crankcase to pressurize, even slightly, you will get oil leaks around and past every seal, every possible place.