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 F8 allows air in through a single 1.20mm bleed hole in the side, while fuel flows through a large 2.0mm internal bore. This makes it richer than the F11 but leaner than the F6 and F9 family. The size numbers do follow a linear sequence, with larger numbers allowing a richer idle mixture. When switching from one F number to another, experts recommend starting with the same fuel size number.
DCOE idle jets, listed in order from richest to leanest air bleed:
F6 DCOE Idle Jet (Part No. 74819): Single 0.70mm air bleed
F9 DCOE Idle Jet (Part No. 74822): Single 1.00mm air bleed
F8 DCOE Idle Jet (Part No. 74821, this jet): Single 1.20mm air bleed (large fuel bore)
F11 DCOE Idle Jet (Part No. 74824): Single 1.20mm air bleed (smaller fuel bore)