At idle I can feel a slight vacuum if I hold my hand over the rear carb. Pulled it apart and could see light around the butterflies. When I loosen the screws and reset them so no light comes through they are slightly stuck and after opening them I can see a little light when closed. It idles O.K. so is this something I need to worry about ? I installed a new shaft (operator error on the old one) but used the old butterflies, new butterflies? could I gently polish the barrel?