Do you have the Holley bronze fuel filters and springs installed in the inlets? You may have something in your gas tank? Even a blush of rust in the gas that gets on your needle and seat can cause them to stick. If it's only the middle carb, then it may be isolated to that fuel bowl? If it's from the gas tank, all three will be susceptible. If it's rust, then you may see a sign of it inside your fuel bowl.