The emulsion tube holds the main jet and the air jet on DCOE, IDF, and IDA dual-throat carburetors. Also fits 45 MCHH single-throat. Two required per carburetor (one for MCHH). Sold individually.
Weber emulsion tube numbers were assigned chronologically, as each design was introduced. It is not linear (a smaller number does not necessarily mean a leaner or richer mixture). The F7 will richen the low end and initial throttle response. The F2 will lean out the low end and initial throttle response (F11 and F16 also lean out the low end, to a lesser extent). The F9, F11, and F16 are the most common sizes. F2 and F7 are recommended when using alcohol fuel.
Note that the 40/46 IDA (Triple) carburetors use a different Weber Emulsion Tube (Part No. 1585), which is held in place by the air jet.