I was getting ready to sell all my bikes and become a peanut farmer. I started going through all the bolts on the carbs and discovered that the 3 bolts and 1 nut on the connecting shaft for the carb lifters were loose! They either worked themselves loose or I forgot to tighten them 2 years ago... oops. That also explains the horribly wonky idle I've always had. Basically the lifters could move a little on the shaft and that would severely affect idle and sync. Once I tightened those up it became much easier to sync.
After all this, the low speed chunk is still there. I'm starting to think it might be inside the gear cluster/final drive sprocket area.
Finally, I had that horrible wobble of death on grated roads to figure out. I've gone through everything and added a fork brace, so it has to be the square profile front tire. I recently got that Duro and decided to put it on and try. You can see how much rounder the profile is.