Carburetor adjustable main jet assembly (optional on late models)

The orifice in the “fixed” part of the adjustable main jet assembly is almost twice the diameter of the orifice in the current fixed (non-adjustable) main jet in standard late model carburetors. This gives you control over the richness of the mixture, from total shut-off (needle part of the assembly all the way in against the orifice of the fixed part of the assembly) to extremely rich, as the needle is withdrawn from the orifice of the fixed jet.
(FCAR_07_58)