Make sure the linkages for the end carbs are doing their jobs. They are supposed to close the end carbs when the center is at idle. If there is any slack in them they may not be fully shutting the end carbs. Disconnect the end carb linkages and manually force the carbs closed. Then set the center carb idle. Reinstall and adjust the end carb linkage rods so that they solidly close the end carb butterflies.