If your carb is running great and there's a sudden change, chances are you've got a fuel contamination issue.

Although a major hassle, sometimes its best to just remove the entire fuel system from the tank forward (tank, pickup, fuel lines, fuel pump, filter, carb, etc.) to either restore or replace each item. T

The culprit these days is most commonly bad fuel which can solidify into small chunks when it sits for long periods of time causing all sorts of problems. It can also be rust and scale, etc. Once its all cleaned up you shouldn't have further issues.

The single air bleed screw is not a good design for fine tuning which is why it was short lived (1968 and part of 69). It basically balances the air supply to each side of the carb (also notice how it turns in the opposite direction). The trick is gaining access to the two lower mixture screws (which they all have, usually hidden by a factory installed lead plug). If you remove the plugs and gain access to the small screws you can adjust them, or replace them with two normal mixture screws and set springs allowing you to make finer adjustments just like later versions. .02