You wanna set the idle mix screw by screwing it in all the way, bottoming out gently, then unscrewing it 1 1/2 turns. That is a good place to start before tuning it.
I would say you found your culprit as to why your truck isn't idling.
The idle mix screw was setup like that when I rebuilt the carb, so it was set like that when I tried to run the engine.
It was the idle screw that was in all the way, and as far as I know(which ain't a whole lot) it would only bring my idle up, not cause the engine to backfire / stall.
