Holley Needle & Seat Assembly 1018-135 | Holley offers a number of needle and seat assemblies for its carburetors. The configuration of the needle and seat assembly and its seat size depend on carburetor application, CFM size and type of fuel bowl used. Operation is simple. Needle movement is controlled by a metal tang on the float arm. Upward movement of the float causes the needle to close the seat; downward float movement causes the needle to open the seat. Seat size determines how much fuel can flow at a certain pressure. A bigger seat size will flow more fuel; a smaller seat size will flow less fuel at a given pressure.