[Marshall V. Daut]

I assume you have a Zenith carb, which the Model A came with from the factory? No matter what carb is on your car, though, it's dirty inside and needs to be disassembled and completely cleaned, especially the idle areas. Either drill out the plugs and clean the passageways or try using the paper clip tool to check passageways. Using compressed air, blow out all orifices and each jet. Your breath is not strong enough to dislodge junk inside the passageways. High pressure is your friend here.
If it's a Tillotson, it still needs to be disassembled and cleaned, but additionally the mounting flange will most likely need to be leveled out with a good file. It warps in the middle and sucks air inside through this "saddle" low spot, which throws off the air-to-fuel mixture. Of all the Tillotsons I have reluctantly gone through for friends and customers over the years, I almost always end up drilling out the lower passageway because it is easily clogged. It's easy to reach and drill out, followed by a smaller drill bit turned by hand to clean out the junk. I usually end up drilingl it out at slow speed. That really turns the carb's performance back on.
Marshall