Changes in the carburetors for 1978-80 cars have made the adjustment of idle mixture impossible without the use of a propane enrichment system not available to the general public. Backing out the mixture screw, of itself, will have little or no effect on the mixture. Most 1979 and later carburetors have mixture screws concealed by staked-in plugs. Mixture adjustments are possible only during carburetor overhaul.
On these models the air/fuel mixture is controlled by the electronic control module of the computer command control system. No adjustment should be attempted.