This is as much an answer to your questions as it is to DetMatt's response below. The passage in the intake manifold is intended to run hot exhaust gas through the intake to speed up engine warmup. When the choke is improperly set, especially in the winter time, there's a lot of unburned gasoline that's part of the exhaust gas flowing through the intake and it settles out and slowly plugs the crossover passage. Sort of like hardening of the arteries. So while the passage is now getting more and more restricted, it's still flowing some exhaust gas and it gets all of that carbon deposits really heated up. Even if the carbon eventually blocks the passage completely, you're still going to have exhaust gas heating everything up and the carbon just acts like a heat soak, that very, very slowly gives up the heat it's accumulated. So when you've been driving for any length of time and everything is up to temperature, the engine can deal with the passage being blocked off. It can also deal with a restart from hot in a very brief period of time. But give all that heat time to boil off the gas in the carb, and you've got what you're dealing with now. All that carbon has gotten really hot, gotten the intake really hot and the carbon gives up its heat much more slowly than the cast iron would cool off, if the passage was open. Also, the lead in older gasolines helps with the plugging up. Everything else being equal, the unleaded gas won't create this mess as quickly. I know, I know, everybody hates ethanol and so do I. But your car was meant to run on premium or close to it, and modern premium gasoline has far less ethanol in it than the current "regular" grades, so once you've got the passage cleaned out, it'll stay cleaned out longer.
What DetMatt is talking about is having a manifold gasket that blocks off the exhaust passage entirely. If you lived somewhere that's warm all the time and a "cold" night goes down into the 65°F area, the car can deal with starting up in that, because for the engine that's not really "cold". But if you live somewhere where the choke stat is going to actually try and close the choke because of low temps, then you're going to have a whole new problem of the car being a ***** to start and keep running without any carb heat from that crossover passage. In cases like this, the engine will eventually warm up (it'll take a whole lot longer and you'll burn a whole lot more gas until it does) and then it will be getting the carb heat from convection coming straight up from the block.
If you're running the stock exhaust manifolds, it's crucial that the heat riser valve in the exhaust manifold is working properly. If you can turn it by hand, it's working OK. If it's stiff, you need to soak it in penetrating oil (NOT when the engine is HOT!!!!!!!!!) and work it back and forth. Also use a small hammer and tap the shaft back and forth gently to free it up. Chrysler used to sell a really, really good heat riser solvent, but the stuff made the EPA go nuts, and Chrysler had to discontinue it. I still have a little bit and only use it when really necessary. If you have a "Krown" oil spray dealer in your area, their penetrating oil works really well also.
