okie wrote: and where is Brian
Right here.. and without more data from the O.P. your thoughts are just about spot on - carb problem or air leak is most likely - I could not add much more other than probably his best best is a new carb. Those rotary valve carbs are easy to work with and clean.. until someone takes them all the way apart , removing the rotary valve body, which rarely if ever needs to be removed as there's little more than the low speed adjustment (which if all else is correct, should not need any adjustment) - that rarely gets gummed up as the movement of the needle usually works to keep that circuit quite clean.. and as KE4 mentioned there is the high speed adjustment, but again if properly cleaned , and carb still seems to need adjustment, there's another problem somewhere, so either a bad carb or an air leak.. might could also be a partially plugged spark screen, but that's pretty rare unless using incorrect mix oil.
But, from the symptoms described, its a lean condition, and assuming all the work so far was done correctly, it is likely either a bad carb or a crankcase air leak somewheres.
How poor are they who have not patience. What wound did ever heal, but by degrees? - Iago (Othello Act II, Scene 3)