Carb tuning, larger accelerator pump nozzle?
This is the carb:
Quick Fuel BD-650 SS-Series Carburetor Black Diamond, 650CFM
Same as the one you posted, just black.
I started balancing all 4 corners and result was the same. I've almost closed the secondary with 1/8 turn open, and tuned the front, result was the same.
Talking this out brought back a memory. When I was changing pump cams, I had opened the secondary some and closed the primary to idle at around 750. This is when I saw a minor improvement so this may lend some proof to the primary transfer slot being too far open even at .030 to .035 exposed. On the last test I never opened he secondary air and confirmed it was closed (no slot exposed) before the carb went on.