Positive Crankcase Ventilation

My 67 XL has been idling fairly lopey lately, and the other day I crawled under the hood to investigate.

One of the symptoms I noticed was that, while in Park, I could tap the accelerator fairly hard and make the car nearly stall . . . ALT light flashing on and everything.

With the car running (doing a weird up and down idle), I popped the hood to listen and observe. The first thing that hit me was a hissing noise, much like what you'd hear if you put your hand most of the way over a vacuum cleaner hose.

Looking around, I noticed the oil breather cap (initially not knowing what this is) . For kicks, I pulled it off. Immediately the car started idling stronger. Hmm.

After some research, I believe I've determined that this breather cap (probably original) needs to be replaced. I tried a shelf cap from NAPA, but found it was too small.

Having read up on this system (PCV), I've learned that no, I don't want to just remove the cap and let the crankcase fumes escape.

QUESTION (1): What effect does this, seemingly, dirty air have on performance?

The second issue I uncovered was an interesting one. While it was idling oddly (and weak), I placed my hand over the air cleaner intake, sealing it. The car refused to stall out, which I understand it should. The hissing sound I heard amplified, and even a whistle developed. It seems this hiss/whistle originates on the driver's side of the carb.

QUESTION (2): What are the obvious points I should be checking? Do I have a bad gasket between the carb and manifold?

QUESTION (3): Am I correct to assume that blocking the air cleaner intake should stall the car?

QUESTION (4): What breather cap is accepted as the best (for a FE390)?

Thanks in advance. :)

Thanks for the reply, it is indeed helpful. I replaced the PCV valve and filter cap, but the issue seems to remain. I did, however, locate the hissing noise. One of the vacuum splitter fittings has a cracked cap.

This engine has a few problems, the real challenge is simply isolating each individual cause/effect. :P