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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. When switching from one F number to another, experts recommend starting with the same fuel size number.
