The power valve has no effect at idle.
It sounds like the idle or possibly the transfer circuit is lean, based on your description. Can you tell from the smell whether it's excessively rich or lean at idle? Any clouds of black smoke?
Look here
http://members.tccoa.com/392bird/carbtech.htm& scroll about halfway down to the photo of the carb ,main body and metering block.
In the metering block, be sure 25 is not blocked. Spray some carb cleaner through it until you know it's clear. Ditto 6 and 4. Remove the idle mixture screws first.
Continuing with the aerosol carb cleaner in the main body, be sure 3 is open to the idle discharge port (below the throttle plates when they're closed) and 4 is open to the transfer slots (above the throttle plates when closed). Be sure 6 is open to the idle air bleeds (a brass orfice, one in each barrel).
Has the carb ever been rebuilt? The wrong main body to throttle plate gasket can block off a passage necessary for idle/transfer.
Is you PCV stuck open? That would cause a huge vacuum leak, the effect would be worse at idle.
If we knew if the idle was rich or lean, it would help.
Jim
