See Figures 1 and 2
All vehicles are equipped with a high altitude compensation system in addition to the feedback carburetor system. This system consists of a high altitude compensator, a check valve and a distributor equipped with a high altitude advance system.
Air/fuel ratio at high altitude is controlled by the high altitude system to be about the same value as the one at sea level, by supplying additional air into the primary main well through the HAC. The air to fuel ratio is controlled precisely by the feedback carburetor system to comply with the California high altitude requirements.
In order to reduce HC and CO emissions and to get better driveability at high altitude, ignition timing is advanced by specified degrees at high altitude. Spark advance vacuum signal is sent to the sub diaphragm chamber of the distributor via the HAC.