The main jets provide a path to the idle tubes and they are driven deep into the main castings so cover up pretty quickly as the bowl fills.
Above the idle tubes either in the airhorn or the main casting just above the DCR's is an upper bleed. It acts as a vent to prevent syphoning all the fuel out of the bowl after shut-down.
I've seen folks and even companies building carb mix-match main castings and airhorns and come up with combinations where no upper IAB is present. This doesn't work well and you'd think the bottom plugs were leaking as the carb empties into the intake after shut-down.
Just though I'd throw that out there in case someone has that issues and can't figure out what's going on?.........