Two suggestions:
- Pull idle jets and blow carb cleaner through them and check them by looking through them
- the idle air bleed jets can be easily blocked with dirt from various sources and need checking
To check idle air bleeds you will need to remove the tops of the carb bodies (ten hex nuts) and see if crud is accumulating around them. These are located just inboard from where the idle jets are installed and pressed into the top of the carb bodies. They are brass and about 3/16" in diameter with a little hole in them. Blowing air through the idle circuit passageway will probably clear the air bleed if there is some crud there so see if cleaning the idle jets fixes your issue first.
Check out my comments regarding sources of crud in the fuel bowl from my site:
Weber Carbs Technical