I once fixed an OS LA (same kind of air bleed carb) by just knocking the dirt out of the airbleed hole . Probably not the issue in this case, but worth checking.

It's also possible that you're just trying to idle lower than the engine will. Do you have a tach to check the idle RPM?

When you idle on the ground, do you idle for the same length of time you do while on final? I've seen plenty of guys "test the idle" for a few moments, but not long enough to match what actually happens in the air. If the engine is slowing loading up on fuel due to being just a bit too rich, I would expect this to happen.