these carbs have a de-acceleration valve on the side of them. Small cover with three screws. It is a rubber diaphram that stretches. that is the most likely cause. Go to www.siriusconinc.com and look for carb parts. You probably should get carb kits and the valves for each carb. Take them apart and clean them really good. Don´t mix up the parts from 1 carb to the other