The metering rods are of a step design and are controlled by vacuum and metering rod spring action. At low speeds the metering rod is dropped down into the primary main jets, thus partially closing off the flow of
fuel. At higher speeds and less vacuum the rods pull up from the main jets allowing more fuel to flow. Experiment with different springs in order to remove any flat spots, especially if the engine is running any kind of performance camshaft.
Carter AFB, 4 barrel metering rod springs. Includes 10 springs, two springs for each color. See #17 in the illustration.