It LIMITS rack travel vs rpm. As the RPMs increase with full-throttle so will the fuel via rack advance, but this travel is modified by the ALDA so that there is less fuel if there is less air. It also serves to keep the engine from overfueling at high altitudes (3% loss in atmosphere per 1000' above sea-level) so the NA engine has an aneroid to replace the ALDA.

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Gone to the dark side

- Jeff