Well I have to say that was an ingenious way to test the carbs for fuel. Glad you have isolated the issue.
Have you pulled your idle mixture screw on the bad carb out? I don't think you stated that it was ever adjusted or the circuit cleaned.
Also, Ed asked if fuel flowed freely from the bowl drains. I'm concerned about the needle valve that is attached to the float. If it is stuck closed, no fuel will flow. You did state that there was fuel in the bowls but just making sure.
