Understanding Carb theory and throttle valve cutaway.

[The Great Jashu]

In regards to the throttle valve cutaway.
-What does this mean?
-What controls this?
-Does it have anything to do with idle air? If so, when does that cutout?

[gixxer6]

Each throttle valve (the round slide) has angled cut section on the bottom intake side.  If you look at the bottom of the slide there is a number stamped, the larger the number the more that is cut away... 

[Checkmarks]

Jashu,

The throttle valve has its affect on the fuel that comes out of the needle jet and jet needle only at the first five millimeter of throttle valve lift (1/8 throttle position).  The cut out affects how much venturi effective vacuum and thus the amount of fuel that is pulled out of the NJ/JN tube.

None of this has to do with idle air.  Throttle valve cutouts have nothing to do with idle.

Imagine a perfectly idling motor.  As you push the accelerator the motor falls on its face, coughs and dies.  Perfect A/F ratio at this engine rpm is either accelerate or die.  When you have the correct TV cutout in there the motor will climb in rpm.  Too lean or too rich and the motor will probably just die.   No chance of melting pistons here, just does the motor pick up in rpm or conk out.