carb adjustment

I have an 84 3/4 ton two wheel drive that won't pass emmissions. How can I adjust the carb to make it pass? Shops here want me to buy an new carb stating they can't make adjustments because the adjustments are sealed at the factory.

The mixture screw is sealed at the factory with a metal cap.
You can use a hand drill to make a hole in the cap (but dont't drill into the screw) to be able to pry the cap off.