Air bleed idle circuit
 

How exactly does the air bleed idle circuit work? Mine I have to set to where it whistles.

It works by more or less having a hole that goes from "upstream" of the throttle body to "downstream" of the TB. On a 2.0, this is made by drilling two holes at an angle into the inner wall of the TB bore. Look down the throat of the TB and you can see the upper one. It actually goes all the way through, but the TB gasket blocks the lower end of it. The second hole is drilled in similarly from the bottom, but does not go all the way through.

In the middle (roughly) of these passages is a threaded hole for a screw. That's the idle adjust screw. When you screw it in, you are blocking more of this passageway. When you unscrew it, you are blocking less of it. When you block more, less air goes through and the idle RPM drops. When you block less, more air goes through and the idle RPM goes up.

I'm not sure exactly what causes the whistle, but lots of 914s do that.

--DD