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13 The mixture screws for ensuring the right ratio of fuel to air are located on both sides of the metering block. Adjustments are made in quarter-turn increments, per the instructions outlined in the main text of this story. Each turn of a screw must be matched to the complementing screw on the other side of the metering block. With a vacuum-secondary carb, these adjustments are only required on the primary side, while double-pumper carbs require the adjustments on the front and rear metering blocks.