I once had two carbs stop working, and the problem was a very strange, but not the first time something like that happened! One of the two vent lines got plugged by an insect making a home in it! Each vent hose is used by two carbs. What happened is the bowls got emptied, and when the fuel pump tried to refill the bowls, the trapped air built up pressure, and stopped the fuel from entering the bowls!!!
So, as a quick test, try blowing through both of your vent lines and make sure they are not plugged. This may not be your problem but this has happened more than once...