No carb back then did that, basic engine condition is what does that, the better the ring and valve seal the better the engine tolerates the load with less dropping down and lugging. Even the latest MPFI engines with IAC control have trouble once the engines start getting older, it becomes more and more of a problem. I'm assuming the engine runs smoothly both with the a/c on and off, just too low rpm with it on. If not, carb not working right. I have two 200K Ford zetecs with a/c and they both now vary more than I'd like when the a/c kicks in/out at idle, the nature of the beast, small engines don't have the idle tickover inertia bigger ones do and once old to lose the ring seal no oomph' to do it either. Even electric fans kicking on affect the idle and the IACs work perfectly.

The very latest carbs (beginning around '80 and higher, look at the California models too) had a basic idle kickup ability, you identify them by the very big vacuum booster on the carb linkage. It is very basic though and you will still have some trouble if the engine is older as it too is based on good ring seal and how the vacuum is high enough with it to work the throttle linkage. Good ring seal= good idle vacuum , with it lower the vacuum drops like a rock and so does the idle assist. Those idle kickup vacuum controls may well be like most of the other detailed late model carb emissions parts or unobtanium now, they were only used a year or two before Ford realized carbs were not the future and dumped all that to go to true FI. With that in mind the parts makers did not make very many parts to cover those years when things were changing by the minute emissions wise. Very hard to find lots of emission parts around then, they are long gone.

Don't confuse the common idle kicker electrical solenoids with that ability, they can only kick up once to stay high as you hit throttle to unload it and cannot kick it up AND down repeatedly like needed here by themselves.