Actually, there technically is not an air screw on the stock keihin carb.
There is an idle mixture screw, which I think is what you have in mind, but it is a fuel screw. Difference being you are changing fuel input on the slow circuit rather than air input. So, if you back it out, you will richen the slow circuit mixture, and if turn it in, you will lean the slow circuit mixture.
It is located on the bottom of the carb - engine side and center. Suggestions such as 2 turns out mean precisley what you suggest. Lightly seat the fuel screw by turning it all the way in, then count the turns out from there.
I turned out this screw out a half turn and I can start my 400ex with no choke. This is awesome, because I always had to use it, even in the summer time.