Backfiring - getting frustrated

[CT Jack]

It sounds like you have a fuel delivery problem and also the timing seems to be advanced a little too much. Toward the end of your video it sounded like the carburetor was starting to gasp for fuel. I would replace the carburetor with a rebuilt Zenith and reset the timing. The slight knocking toward the end is probably the result of being too advanced.
Putting Marvel Mystery oil directly in the carburetor is not going to do much good in my view. It's much better to put it in your fuel tank. Make sure your fuel is good. I recently got a bad batch of fuel which caused my engine to abruptly run unbelievably bad.

[Flathead Fever]

Now that I get to hear it I also agree it's a fuel delivery problem. It starts and idles fine and then sounds like it's running out of gas. Valves would not do that. The ignition most likely would not do that. A vacuum leak would not do that. It wouldn't consistently start and idle for a length of time and then start to stumble. Plugged carb jets would be plugged all the time. Fuel leaks, all that kind stuff would be there all of the time. It runs to good before it starts to stumble. Disclaimer, I can always be wrong, but I was right 90% of the time at work.

The fastest way to know is to lightly spray carb spray, starting fluid, propane, or brake clean into that carb while the engine is starting to die. If it straightens out it was running out gas. If that is the case remove the carbs' fuel line, turn the fuel on and check the flow.

What I think is happening is there is restriction that allows the carb to fill up when the engine is off but when it's running the flow is not keeping up with the engine. It is literally a one-minute test to spray a flammable into that carb while its idling and find out if that is what's happening. This is very first thing I did at work on a carb problem. Doesn't matter what carb and what engine. If you think you have a lean fuel problem at idle you give it an extra spray of fuel. if you think it's too rich you yank a vacuum line and give it more air, but you can't do that one on a Model A. Those tests are fast and you eliminate or confirmed the fuel system as the problem on an idle only problem.

[Big hammer]

Popping out of the carburetor inlet sounds like an intake valve not closing when that cylinder is firing. The change in the sound is him moving the phone. Yes the idle is to fast and maybe the timing is off. Let the MMO in the gas tank do its job go for a longer drive. When it’s well warmed up readjust idle speed and idle air.