An air screw on a Keihin PWL26 only effects the idle circuit of the carburetor.
The idle circuit pulls through the pilot jet exclusively. The pilot jet typically can deliver no more than 15% of the overall fuel when the throttle is wide open.
The claim of 15% is based upon the inside diameter of the pilot jet and the inside diameter of the main jet.
The air screw, as you probably know, will effect mixture at idle primarily. If you are having rich conditions higher up or off idle then you need to take a look at your needle and main jet.
3 on the diagram below is your tapered needle, it sits in the throttle valve(#2) or slide, depending on who you talk to. The jet needle is known to primarily effect mid range mixture while the main jet on wide open effects mixture under those circumstances.
There are clips on older models where you can raise or lower the tapered needle. Raising the tapered needle allows for more fuel to enter the venturi and lowering it removes fuel making the mixture leaner. The needle is moving up and down in the atomizer inside the venturi, at the bottom of the atomizer is the main jet. Unless you've done a lot of mods to the bike up to and including a high performance pipe, you should not have to change the main jet. You can just raise and lower the tapered needle until you achieve the proper level of lean and enjoy.
I couldn't find a diagram of your carb, but this is close enough for the discussion.
Diagram of a De'Lorto Carb
1 - air intake
2 - throttle valve
3 - tapered needle
4 - atomizer and needle jet
5 - main jet
6 - starting device
7 - venturi
8 - idle speed adjusting-screw
9 - idle mixture adjusting-screw
10 - starter jet
11 -idle jet
12 - float chamber vent
13 - fuel inlet banjo union
14 - needle valve
16- float chamber