High Altitude Compensation (HAC) System: Operation
At altitudes above 4000 feet, the low atmospheric pressure allows bellows in High Altitude Compensation (HAC) valve to expand and close off port "A". See Fig 1. Vacuum from intake manifold acts on HAC valve diaphragm and passages are opened between carburetor and atmosphere. Air flows through the HAC valve into the carburetor and leans out the air/fuel mixture. Sub-diaphragm on dual diaphragm distributor will control vacuum advance under these conditions.
At altitudes lower than 4000 feet, the higher atmospheric pressure opens port "A" in the HAC valve. Air passages from atmosphere to carburetor are now closed. Air/fuel mixture is not leaned out by HAC system under these conditions. Vacuum advance will now correspond to actions of the main distributor diaphragm.