Altitude Compensator: Description and Operation

Fig. 57 High-Altitude Compensation System (Federal):

FEDERAL VEHICLES

To meet Federal emission requirements at all altitudes a high altitude compensator (HAC) is added to the feedback carburetor system. The high altitude compensation system consists of a high altitude compensator, a check valve, a vacuum switching valve, and a distributor equipped with a high altitude advance system.

The high altitude compensator maintains approximately the same fuel air ratio at high altitudes as at sea level by supplying additional bleed air to the carburetor primary and secondary main fuel wells through the HAC and vacuum switching valve under the control of the HAC.

At low altitudes the vacuum signal to the vacuum switching valve is relieved to atmosphere via the HAC. Spring force closes the vacuum switching valve preventing bleed air from entering the carburetor fuel well.

At high altitudes the HAC closes causing vacuum signal to be applied to the vacuum switching valve and distributor vacuum advance sub-diaphragm chamber. Vacuum overcomes spring force in the vacuum switching valve and bleed air is applied to the carburetor fuel well. The increased vacuum signal also goes to the sub-vacuum advance causing ignition timing to advance by five degrees. The feedback carburetor system makes final minor adjustments to the fuel-air mixture.

Fig. 58 High-Altitude Compensation System (California):

CALIFORNIA VEHICLES

To meet California emission requirements at all altitudes a high altitude compensator (HAC) is added to the feedback carburetor system. The high altitude compensation system consists of a high altitude compensator, a check valve, and a distributor equipped with a high altitude advance system.

The high altitude compensator maintains approximately the same fuel air ratio at high altitudes as at sea level by supplying additional bleed air to the carburetor primary and secondary main fuel wells. The ignition timing is also advanced by five degrees by the HAC.

At low altitudes the the HAC closes the bleed air passage shutting off bleed air to the carburetor fuel well. At the same time the HAC vents the sub-diaphragm vacuum signal to remove high altitude spark advance. The feedback carburetor system makes final minor adjustments to the fuel-air mixture.