Holley Idle mixture screws non responsive?

Could be because the primary throttle plates are open too far just to keep the engine running. You are actually idling on the transition circuit, not the idle circuit. The idle mixture screws only work on the idle circuit. You mentioned the carb being a Holley...there's a little screw under the secondary throttle shaft to open the secondaries slightly , and allowing the primaries to be closed down and allowing the idle circuit to function again.

Also, check your timing to ensure maximum initial timing.