Been there, done that Brian.
As a commutor bike, this sort of problem bugs the hell out of me so I did indeed put the 30n2.5 from my etz250 onto the bike in question and well.. it was perfect. Reved cleanly and smoothly and it was great.
The ultimate test would be to put the 30n3 onto the etz250 and see if that also gets 3-4k stutter. But it’s fairly conclusive.
I would like to try a replacement original 30n3 carburettor slide before , if anyone has one spare they would like to sell before I blame the carburettor inner bore perhaps…
I know a lot of people blame wear on carburettors, but who else has a genuinely well working original carburettor with 50,0000 miles on it? It seems odd, because there is scoring on the slide sure, but it’s not too bad- and it doesn’t exert much play.