Does the carb bowl really get empty? If so there must be a hole (no kidding) or rather a crack or maybe a warped gasket surface below the bowl at the cast iron carb base. Or perhaps a little ball check has been lost or not in its proper place.
Find a good parts illustration(I think there might be some on this site?), or in a rebuild kit, and look for all these ball checks-may be the one in the accellerator pump. Most of them have a fine gage wire spring that loads them in place, They usually stop fuel flow and they are soooo easy to lose (the ball checks are easily lost too-they look like chrome b-b's). Dont ever think you can take a carb apart while sitting under a tree! These are the pieces that cause a wave of panic when a carb is opened up and turned upside down (where'd that come from??), if you can even find them in the dirt. Been there, done that... (etc).
