I know the problem is with the End carbs. But, Charlie reworked both of them at the throttle plates, are tightly closed. So there is nothing wrong with the center carburetor. So trying to figure out if it's vacuum leak or what.

To check vacuum Leaks use an unlit Propane Torch or what we up North call "Quick Start". Basically Ether in a Spray can.If U notice an increase in RPM  thats usually a Leak sucking in Air.

  

Love your car's..

Sold the Sedan when I bought Convertible. Wife wants to park in Garage in Winter. Not sure Why???

  

My Thunderbird is a "wife" compromise as well.  My wife wanted a convertible.  I wanted a 50s Y-Block hot rod.  Works for both of us.