2 hole idle system (idle discharge holes). The 2 holes are used depending on where the throttle valve is positioned. When the valve is closed the bottom hole is used. Once the valve starts to open, then upper hole will be used. The 2nd hole simply add more air fuel when needed.
Turning the idle adjusting needle clockwise makes a leaner mixture (more air). Counter clockwise make a richer mixture (less air).
Fuel travels from the float bowl to the idle jet using the idle fuel passage.
Fuel is metered by the idle jet and mixes with air from the permanent air bleed just above it. The air/fuel mixture travels down the passage to the idle discharge hole.
The Zenith 28 works just the same except the idle mixture screw it at the top.Don’t remove the idle jet.
Engine Doesn’t Idle
This assumes the electrical system is up to par. Make sure the idle fuel passage is open. Make sure the idle discharge holes are clear. Make sure the passage from the air bleed to the adjusting needle is clear. Check for vacuum leaks. Are the throttle plates in correctly with any taper going towards the top of the carburetor?