Pair of Fully Adjustable Carburetor Metering Blocks for Holley 4150 4160 4bbl It is for a pair of fully adjustable carburetor metering blocks compatible with Holley 4150 and 4160 style carburetors. Fully adjustable metering blocks allow you to stay in complete control of your carb's fuel delivery. Improved throttle reponse over stock, fully tunable for street and strip applications. Calibration upgrade without sacrificing street vacuum provisions.