Idle Jet to suit Weber Carburettors;
IDF, DCNF, 28/36, DM-DMA, DCA, DCNFA, DCNV
Genuine Weber calibrated part
Idle jets supply fuel at low engine speeds and part throttle until the air speed increases sufficiently as to start drawing fuel from the main circuit. This can be anywhere up to 3000rpm depending on the engine and the venturi size used.
The actual idle mixture (while the car is sitting stationary) can be adjusted with the idle mixture screw, however it is the progression holes that play an important role during the "transition" stage from the idle circuit to the main circuit. As the throttle plates open, more of the holes are exposed releasing fuel into the engine. The progression holes are not adjustable unless you start drilling them (not advisable). The progression holes form part of the idle circuit and are fed by the idle jet.
A very basic rule of thumb is that if the idle mixture screw is less than a turn out, the jet is probably too large. If the idle mixture screw is more than 3 turns out, the idle jet is probably too small. Obviously there are going to be many exceptions to this.
Larger venturi will often require larger idle jets to compensate for the lower air speeds an low rpm and part throttle openings.