ORIGINAL: gaRCfield
Try pulling off the carb and putting it back on tightly before screwing it on. Sometimes air can get around the o-ring seal in an improperly seated carb.
After you remove the carb, clean the neck and draw bar / pinch bolt then use a little Silicone sealer when you put it back together. (just a thin film of it wiped around the carb neck, and the bolt is all you need )