First, go take a read here, this is absolutely not my own original idea, I take zero credit, but I think it is important to share:
Maybe everyone already knows this, but it is something I have been struggling with for a long time, and could never find a solid answer:
For a high-strung 2-stroke with a peaky power curve (frankly most mopeds don't fit in here, I'm talking 180deg or more exhaust timing, 25deg or more blowdown), it's obvious that fuel requirements are vastly different on and off the pipe. You can easily make 2x or 3x more torque on the pipe, meaning you're trapping 2x or 3x more air/fuel mix in the cylinder. In my mind, for the longest time, it never made sense that a single carb circuit (main jet) could fully account for this change in required fuel.
This idea that needle tip vs atomizer diameter controls WOT at low speed and main jet controls WOT at high speed was the first I'd heard of speed-dependent jetting. Any tutorial or carb manual I've seen only mentions the impact of different circuits based on throttle position.
I can't say I fully understand the dynamics that cause this differentiation of main jet and needle annular area between low speed and high speed at WOT (my only guess is that at low speed, the atomizer is full of fuel, so needle orifice is the limiting factor, but at high speed, it gets sucked dry, so main jet limits the flow?), but I certainly trust Wobbly and Frits.
The PHBG manual itself *almost* addresses the subject, in the section where it shows how to calculate/check that your needle opening isn't too small (comparing needle annular area to main jet cross sectional area). For most mopeds this is definitely not a problem. With atomizers around 2.6mm, needle tips at 1.4mm, and main jets usually around 1mm or less, there's a few hundred percent more needle area than main jet area. So it's more of a matter of the opposite, I think: making sure you don't have too much needle flow area that if you jet for WOT at high speed, you get way too rich at low speed.
In fact, it's hinted at in the very first post of that enormous KiwiBiker thread: take out the main jet altogether, and it will still run at WOT, at least at lower RPM. I didn't try that, but I made a big change, and tried a W11 needle vs. W9 (1.8mm tip vs 1.4), and went much richer on the main jet (69 to 80). It was a huge improvement at low RPM WOT: it leaned out and got a lot more heat in the pipe, and made a whole lot more power. A little too rich when it revved out. Overall much better than the previous setup.
I will keep experimenting (hopefully with actual data for comparison), but in the meantime, thoughts? Does everyone know this already? If so, I think it should end up in the Wiki. (edited)