In my experience, with the idle adjust screws completely unscrewed the engine should barely if not even idle(ie. the throttle plates are basically fully closed and there isn't enough air/fuel mixture to run the engine). It seems to me that that front carburetor is letting a ton of air (and fuel) into the engine which suggest that the throttle plate at the back of the front carb has a problem (bent, wrong part, installed incorrectly, who knows) or that something in the linkage is holding the throttle partially open (bent part, incorrect installation). I think I'd start by removing that front carb and checking the throttle plate to make sure it's installed correctly and closing fully. It's easy to take off, just remove the choke cable, fuel line, and 4 screws that hold it to the intake manifold. If that looks ok, I'd start looking at the throttle linkage. You should be able to remove the dashpot on the front carb and with a flashlight and maybe small mirror you could make sure that the throttle plate on the front carb is closing completely in the "no throttle" position.