Holley Idle mixture screws non responsive?

A 300-400 drop in rpm is pretty big IMO. If you have a big cam and tight converter, it can cause issues. Check for that advance being pulled out in gear like I mentioned before. Have some one stand on the brakes and drop it in gear so you can see what it's doing with your timing light.

Check/adjust the float levels too. That's another first step. If they are too high, it may be dripping fuel into the engine.

Bump the timing up to 18-20 and see how it runs. Or, get it warm and keep advancing the timing until it kicks back on the starter, then dial it back a few degrees and see where you are at.

You really need to get the initial timing issues squared before worrying about carb idle adjustments.