The popping seems to indicate your timing is too retarded. What is your timing at idle? Most engines like about 12 degrees BTDC for a mild engine, but I like to add a degree per 1000 feet altitude. This is kind of a general rule of thumb.
So when it's idling, check it with your timing light and see where it's at. If you're not equipped to check timing, then advance your distributor a bit at a time.
So back to your problem. You open the throttle when you try to drive it, and it dies. Yet it idles okay. Your idle circuits work. But how about your accelerator pump? (Depending on what carbs you are using). When you are idling the engine, what happens when you suddenly open the throttle by hand? It dies, right?
Okay. Is there a strong smell of gas when that happens? If so, probably too much fuel out the squirters.
Otherwise, perhaps it's exactly the opposite. Not enough fuel will cause it to die also, especially under a load.
I suspect your accelerator pump circuits.
But why are your plugs sooty? Well, at idle, you're definitely rich. Of course, you've got two carbs, so twice as much fuel is going to your engine at idle. I'd cut the air fuel mixture way back to bring that back. With two carbs, you might try about 1 turn out from completely closed on your idle mixture screws, and if that idles, bring each idle mixture screw more closed a little bit at a time. Like maybe an eighth of a turn. If you're on the right track, your idle speed should pick up. If you've gone too far, the idle will start stumbling and might die.
Sorting out your engine with a single carb first is a very good idea. I think it'll still run on all cylinders on a single carb. This is simple. Just leave the second carb on there, and disconnect the throttle lever to it temporarily.
One thing that really concerns me is, if this is a new rebuild, did you even break in your cam yet? All this fartin' around at low rpms can ruin a brand new cam. It's imperative that you immediately pop the rpms up to a fast idle of 2000 rpm for about 20 minutes. If your engine can't support that, your cam could be flattening its lobes. So if you haven't broken your cam in yet, as soon as you can get it to take a fast idle, worry about that first. Your idle circuits work, so that should be possible.
