I got off my butt today and went out to work on the HD. Whereas the Zamas have a long tunnel that runs towards the idle port that lines up the bubbles in the idle circuit via capillary action, the HD has a bigger and odd shaped cavity under the welch plug.
The Zama 'freight train' of bubbles ensures a more uniform idle mixture to the idle port. The HD has various size bubbles that shift all over the place. The Zama idle circuit is far superior and allows a better idle (and when you adjust the 'low speed' needle it only adjusts the idle mixture, not the entire low speed mixture).
So I decided to try and redesign the idle circuit on the HD. I found a small stainless steel tube that was approximately the ID size of the Zama circuit. I could only get about 5mm of length in the HD cavity and set it in place with some of the local 'JB Weld' substitute. While I was in there I opened up the forward air bleed/transition hole to .55mm.
After pizz-revving the saw in the backyard, it appears to have a better idle and a better throttle response. I'll have to put it in some wood and give it a hammering to really check it out, but it looks good at this point.
Good thing I'm retired, it took about 4 hours to do.