So all Federal vehicles will meet the federal requirements at high altitude, all vehicles using a carburetor are equipped with a high altitude compensation system, in addition to the feedback carburetor system. The high altitude compensation system is made up of the following components: a High Altitude Compensator (HAC), vacuum switching valve and a distributor equipped with a high altitude advance system.
Air/fuel ratio at a high altitude is controlled by the HAC to be approximately the same value as the one at sea level. By supplying additional bleed air into the primary and secondary main wells through HAC and vacuum switching valves, the correct air/fuel ratio is obtained. In order to reduce hydrocarbon and carbon monoxide emissions and to get better performance at higher altitudes, the ignition timing is advanced by specific degrees at specific altitudes. The spark advance signal is sent to the sub-diaphragm chamber of the distributor, through the high altitude compensator. On California models, instead of using the vacuum switching valve, a check valve is used in its place.