Zenith 228 Idle Circuit
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?
DIFFERENCE BETWEEN 28 AND 228 SERIES
The basic difference is in the idle circuit.
The 28 series has a single idle discharge plug in line with the closed throttle plate. The idle needle valve is located in the air intake body.
The 228 includes a two hole idle system with one hole located above the closed throttle plate and the other below the plate with the idle needle valve located in the throttle body.
