Has this been a problem since you bought the car, or did this problem just start? The way you phrased your first sentence, I would assume the car came with this problem. That means the previous owner couldn't find the source, either, and simply sold the car.
He probably had tried all the things you have, likewise without success.
While reading the list of symptoms and what you did to correct the misfiring and backfiring, like post #8, I suspected the carburetor. Possibly a passageway inside the carburetor is plugged. That would still allow a nice idle, but when more demand for fuel is needed, the plugged passageway will not flow enough fuel = misfire. I know some guys have had success just blowing out passageways with compressed air, but I have also encountered Zenith carbs (and most Tillotsons) that needed the lower passageway to be drilled out because of accumulated crud. No amount of compressed air or soaking in carb cleaner will get rid of this crud. The brass plugs must be drilled out and a wire, drill bit or small brush inserted into the passageways to "scrub" them clean. It has already been suggested to try a known good carb. If you have a local Model A guy with a carb he'll loan you for a temporary swap, go that route. If no improvement, then at least you can rule out the carburetor and concentrate on other areas.
Does the car have an auxiliary water temperature gauge or oil pressure gauge, perhaps anchored at the bottom of the instrument panel? A strange question, I admit. But one with a reason for asking. I restored a 1930 Coupe for a doctor friend in Phoenix 20 years ago. After I moved away, the doctor died and the family sold the car. It showed up here in Iowa - but with a constant misfire/backfire that wasn't there when I gave the good doctor his finished car. The place that now owned the car asked me to find the misfire. I spent a great deal of time trying to track down the source of the misfire. Of course, the usual suspects were checked out, but nothing helped. Like you, I was really becoming frustrated. Then I noticed something that I had not installed when restoring the car. The doctor had put in an accessory water temperature gauge mounted at the bottom of the instrument panel. There was an electrical wire running to the terminal box for power to operate the gauge's light. While re-installing the instrument panel for umpteenth time in an effort to find the source of the misfire, I neglected to hook this wire back up. Upon starting the engine and driving the car, NO misfire, NO backfiring! Hurray, I thought. I noticed the wire's connection dangling, so I re-connected it. Guess what!? Yup - the engine started misfiring again. Aha! Disconnecting the wire again resulted in no misfire. For some reason that only an electrical guru could probably explain, connecting that water temperature light wire was causing a misfire in the ignition system. Everything happens to me.
I am not saying this is your problem, but if you truly have tried everything and the misfire remains, before dumping the car on Craig's List, disconnect any auxiliary wires to accessory electrical items. Don't give up! There IS a reason why this is happening and it is probably a very simple thing. Once you find it, you'll kick yourself for not thinking of that right away. But you will also come away with one heck of a feeling of satisfaction that you stuck it out and solved the problem. That is priceless.
Marshall
