Why? Is it not running right? Should be in the range of 1 to 1.5 turns on both screws. Anything much outside of that, and you're trying to tune around another problem. Intake boots are very prone to cracking after 25 or so years, and that will cause what seems like carb problems. Ignition is another possible area. Also, the carb itself can have hard or cracked diaphragms in it, or the fuel line might be rotten or collapsing under vacuum. We need more info.