You hit the nail right on the head, sometimes when the primary idle circuit is too rich and the mixture screws are unresponsive this can be a great remedy...look for the idle transfer slots on the primary side to be covered or the carb will think it's not at idle and run rich,this is true on most carbs.drilling the butterflies to correct this problem is a last resort and I don't recomend doing it till all other adjustments have been corrected. on the demon with the idle ease you still have to adjust the secondary throttle plates before the idle ease system will funtion properly,with the demon carb you need to read the instructions and adjust everything before installing the carb, contrary to popular belief they need all adjustments checked and the instructions followed and then you have one fine carburetor....This Is all my opinion and ya'll can do what you wan't with your carb,after all it is your carb....do as you will :cheers:
Holleys have an adjustable secondary throttle stop. It is possible to open the secondaries slightly at idle so the primaries may be closed farther. This is sometimes needed with long duration cams.