I'm with Stan. I've always started with the air bleeds fully closed and then open one on a low (front to back) cylinder just to balance that side. Never heard the folklore about air bleeds causing a flat spot.....That is usually caused by something entirely different.
I have also made a manual throttle similar to Danny's, but mine is a rod attached to one of the case studs near the distributor. The rod has a turnbuckle on the other end and a way to clamp onto the throttle cable arm on my (gasp!) heim-jointed, hex bar linkage. Once assembled, I can dial in whatever engine speed I want by turning the turnbuckle to get me to 1,500 - 3,000 rpm to make sure things are OK on the mains. I always disconnect the linkage at idle to sync things there. After that, I use the same process as Danny and Stan.
Looks like you found the culprit in that opened air bleed. Those little buggers can make a big difference. That's probably why people recommend to close them unless you need to balance both throats on the same carb - that's what they're there for, but 12 to 18 is a big difference on one carb. As mentioned, I would turn them all in closed, then only open one on a low throat on the same side carb to bring it up to sync on that side.