If the spark timing is too low, the idle has to be opened up so far to get it to idle that it pulls fuel from the boosters and not the idle circuit.
You have to remember, you only have so much room for the butterflies to be closed enough to run off the idle circuits. If the timing is set too low then the further the idle speed has to be opened to compensate. When it's off the idle slots in the base of the carb it pulls fuel from the boosters to idle, and this makes the idle mixture screws unresponsive. Thats why I need to know what the base timing is set at and if the vacuum advance is in use.