That sounds like one (or maybe both) of the carb diaphragms might not be seated properly under the tops and leaking air. That's really easy to check by removing then tops and insuring those sealing ridges are seated in the recessed groves. It can be tricky to get them just right because the material will swell from contact with fuel and then they try to wrinkle when you put them back in. if that's the case they'll shrink back to size overnight if you leave them out of the body in a dry clean place.
