Those are the Progression holes and like everything else, must be perfectly,
absolutely clean.
Unlike other brands of carburetors, Webers do NOT tolerate even the
slightest dirt (or water) inside any passage tor jet so "Cleanliness is literally next to Godliness",....
I would also recommend very carefully removing all the lead plugs to get all the passages spotlessly clean. I use old lead shot to reseal the holes when I'm done.