Bummer, problems like this can be challenging to diagnose frown I'm assuming you are getting good response from the mixture screws when adjusting them ? if so that would indicate the idle circuit is doing it's job and functioning as well as compensating for the suspected shaft leak.

I just had another thought When idling, have you tried SLOWLY restricting the air into primary side of the carb? Think of very slowly closing a door. IF you get to a point in which the idle picks up / smooths out it indicates a small vacuum leak that is otherwise not noticeable,
i had one years ago that drove me nuts for a quite a bit, I finally just checked & tightened the intake bolts,
MAGICALLY all was well wink
Keep us posted

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