idle speed or timing. the main 2 reasons, the third is too low of octane fuel.
mostly it's too high or idle speed, o too far advanced timing.
Leaned it out to the max?? You know that will make them run really hot, boiling the gas in the float bowls and probably cause dieselling right? If you dont have a high performance cam your timing should be ~ 8-10 degrees BTDC. Then adjust your idle air screws till you get the highest/smoothest idle rpm and best vacuum, then screw them clockwise equally till you shave off about 50 rpm, this is called "Lean Best" setting. All the idle tuning info can be found in the Edelbrock manual that came with the carb or on their website in the form of a .pdf file. Post back how it goes!
Yupper! IIRC, factory timing is 4*... i would say start there, get the carb tuned, i say get a q-jet, or at least put the holley back on....those Edelbrock/carter carbs are junk. Look for vacuum leakes too, like on the intake manifold gaskets, the easies way to find the leaks on the underside of the manifold, is to use a propane torch with the tip removed, and a hose stuck onto it, and stick it in your oil filler with a shop rag stuffed around it, the disconnect an breaters or pcv valves you may have, and plug the holes in the rocker arm covers, start the truck, and then turn on the propane, and listen for the rpm change....if not, underside of the gaskests are good. you can use carb cleaner on the top.