Federal Vehicles

Fig. 14 High altitude compensation system (Federal):

To meet Federal requirements at all altitudes all carbureted vehicles are equipped with a high altitude compensation system, Fig. 14 in addition to the feedback carburetor system. The high altitude compensation system consists of a high altitude compensator (HAC), a vacuum switching valve, and a distributor equipped with a high altitude advance system. Fuel/air ratio at high altitude is modified by the HAC to be approximately the same value as sea level. The HAC supplies additional bleed air into the carburetor secondary main fuel well through the HAC and and to the primary main fuel well through a vacuum switching valve controlled by the HAC.

* At low altitude the vacuum signal to the vacuum switching valve is relieved to atmosphere via the HAC.

* At high altitude the HAC closes the air-leak line and vacuum signal is sent to the vacuum switching valve. The vacuum switching valve then opens an additional air bleed to the primary main well. In addition the HAC also opens the additional bleed air passage to secondary main well.

* The fuel/air mixture (compensated for altitude) is then precisely regulated by the feedback carburetor to comply with emission standards at all altitudes.

* To further reduce HC and CO emissions and improve drivability at high altitude ignition timing is also advanced at high altitude. The vacuum advance signal is modified by a sub-diaphragm chamber controlled by the HAC.