RE: Walbro WT-71 Carb?
I'm not familiar with your particular Walbro model, but I have experienced a similar problem with Walbro carbs and found the diaphram cover to be warped or bowed. Air leakage at this critical area could be your problem. Check this and ensure it mates perfectly with the carb body. If the cover is questionable, you could resurface it on a flat surface using a fine grit sandpaper (220-400 grit). Hope this helps.