Subtle "popping" typically indicates one or more of the Main-Idle ports is partially blocked - creating a "too lean" mixture. The "Cleaning Tool" I make is a very affective method of flushing out all of the fuel/air circuits.
While you've got the carbs on the bench, remove the idle jets and inspect each one under magnification. If these jets are "originals" I strongly recommend they be replaced as they can mimic a similar situation.
Degraded idle-jets have probably driven some to a point of insanity, trying to figure this out. It did for me until I put a set of 6 originals under a microscope.
Too be clear: If these jets are anything but 'Perfect" - they are garbage - can't be "repaired" - don't waste your time !
1980 KZ 1300 sr# KZT30A-009997
Always High - Know Fear !