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Adjustable Part Throttle

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Q-jets have an adjustable part throttle (APT) screw that varies how far the rods are allowed into the jets, changing the metering from off idle to mid throttle. This is an odd-size Allen screw from the factory, but Murphy replaced it with a Phillips screw (arrow) to cut down on tuning tools. This screw can be accessed by knocking out the factory aluminum plug in the carb top, tapping the hole 1/8 inch NPT, and filling the hole with a thread-in plug.