I'm here. I think that the reason that you don't have any idle screw adjustment is because the throttle blades are too far open past the transition slot that progresses the carb's idle circuit into the main metering circuit. The carb is using the main metering circuit when this happens.
A way to remedy this is to open the secondary throttle blades slightly and then because of the additional air that now allowed into the engine, allows you to close the primaries enough to get the blades on the primaries down to the transition slot. Once the prim. throttle blades are into the slot, the idle circuit should come on line and you'll be able to then use the metering screws. If you still don't have any adjustment, open the secondaries a little more.
The idea is to allow the same amount of air into the carb as was entering the primaries initially. Only now it allows you to close the primaries enough to regain the idle adjustment and maintain the desired engine speed. If this still doesn't do the trick, then check out Edelbrock's web site. I believe that they have detailed info on their carbs. there.
As for the backfire, I think that this is due to the fact that there is unburned fuel entering the exhaust that is being ignited by the hotter than normal exhaust valves caused by over rich jetting and a lengthy exhaust duration. The fuel is igniting as it exits the valves at lower engine speed..
How radical of a camshaft are you using? Has the carb's calibration been changed from the factory settings? A cam with a lengthy duration can contribute to this condition and most carbs are calibrated off the shelf for engines that are equipped with stock or mild performance camshafts. Where idle speeds can be set at stock on near stock levels and the reduced amount of cam overlap and wider lobe separation allows for more complete combustion at lower engine speeds. | 
Sounds like what would be a blown power valve on an Autolite or a Holley. Let me check and see what Edelbrock is using for a power valve system and I'll get with you in the morning on that one. I haven't messed with an Edelbrock for so long that it would be entirely possible for me to remember something being one way, but it's actually some other way. Wouldn't want that. 
