Well, in lieu of a problem due to fuel pressure, I may get a regulator. But my question is, why did it work so good when I had that 750 on it? I've done your test before, and didn't see anything. Eventually, as a quick fix, I just tuned the idle with the secondary rod height.
Also, I realize that the secondary throttle blades don't open because of vacuum, but the secondary air valve doors do operate on vacuum needs, and because of a cam that also lifts the secondary hangar, and therefore the rods, technically the vacuum does have an effect on the secondaries. Hence the fact that they area vacuum secondary carburetor. As far as hangar height I've got about a dozen different hangars, and have experience tuning with them, and I've gone to both extremes, and no difference - I still have no idle adjustment. I've even trued all my hangars, knowing that both sides area at the same height.
The only other thing that I can think of, but I can't see externally, is that the air horn is cracked somewhere causing problems.