Alright OP, hope you've been reading up, because I have a solution (at least for MY lean sneeze).
So after watching several YouTube videos on brazing and soldering with lead free (brush-on flux solder), I decided to give it a go. I brushed the flux on after some rigorous cleaning with acetone and steel wool. I applied heat off to one side so I wouldn't burn the flux, but it still charred and oxidized the brass before I could get it hot enough to stick. And then it would puddle up and fall right off. Tried again, same thing. So what I ended up doing was heating the brass up, clean it off while hot, then apply flux (which still boiled) and while directly applying heat I applied the solder. You have to be quick, otherwise it'll oxidize the brass and the solder falls right off again.
I applied a very light coat, just enough to get coverage all around. I didn't measure it, since I knew it would be uneven. I chucked the shaft up in my drill press and used a medium file to knock down the high spots, then a small hobby file to smooth it out. First measurement was 0.235. Originally the worn down spot measured 0.215", so I added approximately 0.020" of solder (after getting the high spots knocked down). I decided to aim for 0.001" of clearance, so add 0.006" to 0.215" and I came up with 0.221". So away I went with the files and stopped every few minutes or so to see how far I got. Patience is an absolute must here. I stopped when the shaft got down to 0.222". I pulled out the polishing cloth and mirror-shined another 0.001" away. Exactly 0.001" of clearance.
I gave it a go in the bore and found that it was a little too tight. Turns, but not too freely. So I knocked it down another 0.001. Better, but not buttery smooth. Another 0.001 and it felt awesome! I could barely feel any wiggle in the shaft, and it rotated so much more smooth than before. I put it all back together and fired it up.
Here's where I found out something really interesting. The throttle stop on the back (it's a thumbscrew) adjusts the whole throttle linkage. So what that means is that it also moves the timing advance baseplate. As racerone said, if the throttle plate is not fully closed, you can get the lean sneeze. True, but there's more to it than that. That's assuming the baseplate timing is right. So what you end up having to do is back the screw on the forward end of the throttle shaft out and turn the linkage with the timing baseplate fully zeroed out. That way you can adjust the timing and throttle opening independently. Once that's dialed in, the lean sneeze went away completely.
Hope my endeavors have helped at least someone
