This happened to me two weeks ago and I diddled with the carb for a while...Tried everything, richer, leaner, reset the choke but nothing made a difference...it idled badly and didn't 'sound' right. It ran dreadfully at low revs, but OK at above 2500 other than the odd 'miss' when I stabbed the gas hard.
Turned out for me that I had a broken push rod and NO compression in Cylinder 2...
Hope you don't have the same!
Glyn