Look closely at the needle valve above the float and the seat to see if it's sticking. If not, swap the slides. It's a quick way to eliminate the diaphragm as a culprit. Then you might just spray some carb cleaner into the 'T' junction and make certain that it's really feeding the right carb freely. You've done a lot of troubleshooting to get this far and there aren't that many options left.
