Before you pull the carbs off, just disconnect the fuel line at the left carb. I believe you have the vacuum operated petcock in which case fuel will not flow with the petcock in the "on" position unless the engine (or an external source) is supplying vacuum to open the valve. I believe it will flow when in the "res" position (at least that's how it worked on my GL500). That's how you get fuel in the bowls to start the engine so the petcock will work.:roll:
Rube Goldberg, your phone is ringing.