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High-Flow 6 AN male flare brass valve, complete assembly, includes: poppet, spring, and shims, for gasoline and methanol. Since the engine makes less power at part throttle, it needs less fuel. The spool in the barrel valve is the main control for part throttle fuel delivery. At wide open throttle, the spool has a large notch cut into its side to pass all the fuel to the nozzles unrestricted. As the throttles are closed, a tapered ramp ground into the side of the spool reduces the passage to restrict flow to the nozzles. When adjusting the hex-link to set the idle fuel rate, the spool is being re-positioned to make the passageway larger or smaller to the nozzles. If you remove the fitting from the top of the barrel valve, you can see this ramp. Because each engineâs part throttle fuel requirement varies due to camshaft, manifold, and exhaust system selection, the fuel rate the ramp delivers at part throttle is seldom exactly what the engine needs, but the secondary can be used to tailor it.