Well, after some digging, I found this that explains a lot about the snorkel flapper valve.

https://www.allfordmustangs.com/threads/intake-air-for-your-260-289-302.217734/

BobE wrote:

As the link for Ron68’s post indicates, years ago on a 66 Mustang that had issues in cold winter temperatures, we found that flapper stuck in the open position.  The mechanism pivots were rusty, took it apart, cleaned it, applied some lubricant and was good to go.  This car also had the choke stove mechanism that needed to be cleaned and lubed from time to time.
The 66 did not have vacuum assist on the flapper, and I’m not sure how that system functions.  

I've seen themostatic vacuum switches.  They were on everything in the late '70s and early '80s running emissions stuff.  They allowed for example timing and carb adjustments based on engine temperature in an effort to reduce emissions.  Possibly the air door was controlled thusly.  Otherwise I'm not sure how much help vacuum would really be, so it would be interesting to know. 
 

MS wrote:

Didn’t the early ones just have a thermostatic spring in them? As the spring heated up, it got longer, allowing the door to open or close, whichever blocked off the hot air.
Disclaimer: I have only had one car ever with an intact air cleaner snorkel and never gave it much thought. They usually wound up in the trash can in the name of higher performance hot rodding.

The article I found just calls it a thermal actuator - maybe a spring, maybe a wax pellet type, I don't know. And maybe it is PFM  too. Anyway, it works when heated up. I will pull it and clean it up as I got paint all over it. I will then do some more ops checks and adjustments to close up the gap when in the cold position.
 

Welp Ron 68 (I will withhold all my ugly remarks about 'rusting-away in PNW'...or the old wait for the three months of dry/nice weather before driving... or remarks about how "rich-running gas-powered-vehicles" are destroying my planet and you should buy a battery car for humanity....or will you REALLY be driving this Mustang in cold/damp PNW weather..I won't say any of that) 
  Ya know...many Mustangs and SBF came with open element air cleaners and the flapper-damper-hickey-dodger wasn't an issue soooo ya could just switch to that and let the automatic choke and warm air under the carb base handle the situation.
   Like MS.. I usually just wired the damper door open and even flipped the top of the air cleaner so I could hear the 4 barrel "moan" as it opened at full throttle.
Soooo the short answer is.....(This ain't a short answer) ..Beats ME.. I always lived in the sunny South!
   I AM still a big fan of manual chokes which would solve a LOT of your problems with the exception of one thing.....
   I can't remember to close it off once the engine is warm !!!
Soooo sorry Ron 68...there is no answer to this question !!!
6sally6

Last edited by 6sally6 (3/07/2025 12:55 PM)

6sally6 wrote:

Welp Ron 68 (I will withhold all my ugly remarks about 'rusting-away in PNW'...or the old wait for the three months of dry/nice weather before driving... or remarks about how "rich-running gas-powered-vehicles" are destroying my planet and you should buy a battery car for humanity....or will you REALLY be driving this Mustang in cold/damp PNW weather..I won't say any of that)
Ya know...many Mustangs and SBF came with open element air cleaners and the flapper-damper-hickey-dodger wasn't an issue soooo ya could just switch to that and let the automatic choke and warm air under the carb base handle the situation.
Like MS.. I usually just wired the damper door open and even flipped the top of the air cleaner so I could hear the 4 barrel "moan" as it opened at full throttle.
Soooo the short answer is.....(This ain't a short answer) ..Beats ME.. I always lived in the sunny South!
I AM still a big fan of manual chokes which would solve a LOT of your problems with the exception of one thing.....
I can't remember to close it off once the engine is warm !!!
Soooo sorry Ron 68...there is no answer to this question !!!
6sally6

Yes, Mike, I do in fact drive my Mustang sometimes during our cold and wet and dank (a lie we spin to keep the sunny state immigrants out) winters. We actually have a Veteran's Day Parade I participate in, and the weather up here in November is usually cold and damp.
I am just trying to install the correct air cleaner on the car my late brother in law tried hard to keep stock when he did his restoration on it. Once I do the adjustment on the flapper (valve plate) and I get the 40 year old red (should be black) heater hoses replaced, I will do a full ops check of the system and give you a report.