Venturi vacuum vs air flow is a quadratic function. It is not linear. As air flows through the venturi, the vacuum at the top of the needle jet increases exponentially. So, when the air flow doubles the vacuum increases like four times. That makes the fuel mixture go richer and richer as the rpm increases, but you want the air/fuel ratio to remain relatively constant (about 12.5:1). To compensate, carburetors use air correction to try and maintain a reasonable mixture over a wide range of rpm.
The standard PWK and the Mikuni VM use a simple air correction system that does OK, but it fails to achieve a perfect air/fuel ratio over the entire range. Both these carbs go rich at high rpm (when the throttle is wide open). That’s why the aftermarket has come up with third fuel circuits like the “Thunderjet”, or the “Dial-A-Jet”, or the “InteliJet”, or the “Power Jet”.
You can put in a larger main airbleed to try and correct the rich condition at high rpm, but then you end up with a lean condition at lower rpm. You can put in a smaller main jet to try and correct the rich condition at high rpm, but that will also result in going lean at lower rpm. You can fiddle around with the airbleed and main jet until you go nuts, but you will probably never be completely satisfied.
This problem only pertains to WOT conditions. At part throttle, the needle jet and slide needle are takin care of business. But snap the throttle all the way open and those two components are out of the picture. At WOT, you are relying on the main jet and air correction system to deliver fuel at the proper ratio. That’s a tall order for a fixed jet with a fixed airbleed.
The emulsion tube offers a little bit of relief. As the venturi vacuum increases exponentially, the fuel level in the emulsion well is dropping, and more emulsion holes are being uncovered. The additional air correction helps to compensate for the exponentially higher vacuum. It’s not perfect, but it should be an improvement over a simple airbleed system that only admits air at the very top of the needle jet. With an emulsion tube, the naturally occurring drop in fuel level adds more air correction as the rpm rises. It adds another dynamic component to the air correction system.
This YouTube vid does a nice job of explaining the quadratic behavior of a venturi.
https://www.youtube.com/watch?v=BvOqNwNuuAA