If it's setup correctly with progressive linkage you should not be operating the front carb at cruise speed at all, only the rear carb. All 4 throttle plates on the front carb should stay shut at cruise. When you nail it you want them all open and that is what the progressive linkage is for. On a dual carb setup you are tuning for mileage, you probably don't even want the idle circuit to operate on the front carb. You just want to run the whole show off the back carb until you get on the throttle a little...