You can verify all of this from the Weber handbook on the 32ADF: https://www.mirafiori.com/forum/viewtop ... 69&t=96376
Idle operation, summarized: Fuel from the float bowl is drawn through a passageway, infused with air, drawn through the idle jet, down a vertical passageway, and through the mixture screw.
Progression operation, summarized: The increasing angle of the primary throttle progressively shuts off fuel delivery through the mixture screw and begins fuel delivery through progression holes (metered by the idle jet) and finally the primary main jet. When the primary throttle reaches about 3/4 open the secondary throttle begins to open, causing fuel to deliver through progression holes (metered by the secondary jet) and finally, the secondary main jet.
Here is the actual text from Weber:
Other fun facts:
- If the primary idle jet is too small for the motor then the car may run with the screw turned out from fully seated, but continuing to turn it makes no difference at all in operation, it neither speeds up nor slows down
- If the primary idle jet is correct then the car will run with the screw turned out from fully seated and will increase in speed as the screw is extracted further and decrease as the screw is inserted
- If the primary idle jet is too large then the car will run rich with the screw at any position.
"Jet" is really not a good name, as the "jet" is really just a calibrated orifice and isn't exposed to the air stream at all. If you look at an idle jet it has a side hole and a nose hole. Fuel passes through the nose through the side and onto the chamber. All jets - main, idle, air - calibrate fuel or air in passageways, the passageways feed either the mixture screw orifice (primary side) or progression holes drilled into the side of the carb.
Idle mixture is not set with the idle speed screw(s) and rarely are they set correctly. A car without air leaks and a good clean carb car should idle - roughly - on the mixture screw alone at about 700 - 750 RPM, the idle speed screw brings it up to 850-900 RPM.
If you've ever had a FIAT with a failed idle cut off switch then you know exactly what this means - the car runs just fine except at idle because there is nothing feeding fuel to the motor with the primary idle jet shut off.