The idle jet is installed under the jet cover on the Weber DCOE series and 45 MCHH carburetors. The F designation indicates that this jet controls both air (side inlet hole) and fuel (inlet hole at the tip). The F number refers to air control, and the size number refers to fuel flow. Two idle jets are required per carburetor (one for MCHH). Sold individually.
Note that Weber assigned F numbers chronologically, as they designed the jets. They do not follow a linear "richest to leanest" sequence. The F9 allows air in through a single 1.00mm bleed hole in the side, making it leaner than the F6 but richer than the F8. The size numbers do follow a linear sequence, with larger numbers allowing a richer idle mixture. The 50 size is most commonly used as a starting point when changing from one F number to another.
The F8 DCOE Idle Jet (Part No. 74821) is the next step leaner, followed by the F11 DCOE Idle Jet (Part No. 74824). At the other end of the scale, the F6 DCOE Idle Jet (Part No. 74810) is the richest available.