I'm no expert, but from what I've learned, you probably have the butterflies open too far, providing enough signal to start the mains flowing. Air bleed adjustments and other tuning changes may be needed on your combo to fix this. You should NOT have fuel flowing through the mains at idle at all. All the fuel should be supplied by the idle and transistion circuits. If the carb is tuned right, you'll have about .020" of transfer slot showing and the idle mixture screws out 1-2 turns or so.