My first guess: it's a linkage or synch adjustment, at idle one bank is doing all the work. Raise the idle speed screw and/or mixture screw on the "cold" (rea) carburetor. Probably lower it on the hot carburetor.
Maybe I wasn't clear enough - I have just one carb.
With "bank" I mean cylinders 4,5,6 which go to the right exhaust header.
