Altitude Compensator: Description and Operation

Fig. 82 High Altitude Compensation System:

Fig. 83 High Altitude Compensation System Schematic:

FEDERAL AND CANADIAN MODELS ONLY

This system leans out the air/fuel mixture, which becomes richer as altitude increases. The system ensures proper air/fuel mixture by supplying additional air to the primary circuits of the carburetor. The system also advances ignition timing to improve driveability at high altitudes. The HAC control valve contains a bellows that is sensitive to atmospheric pressure.

Fig. 84 HAC System Operations Chart:

As atmospheric pressure drops, corresponding to an increase in altitude, the bellows expands. As atmospheric pressure rises, the bellows contracts. The bellows controls opening and closing of two internal bleed ports, which in turn control vacuum applied to the HAC valve diaphragm and distributor sub-advance diaphragm. At altitudes below 2570 feet, the HAC bellows is fully contracted. In this condition, vacuum applied to the valve is vented to atmosphere. No air is drawn into the carburetor atomization circuits and no vacuum is applied to the distributor sub-diaphragm. At altitudes above 3930 feet, the HAC bellows is fully expanded. In this condition, vacuum is trapped in the valve diaphragm chamber. Vacuum applied to the HAC valve diaphragm opens the air passage and air is drawn into the carburetor atomization circuits. This also allows vacuum to be applied to the distributor sub-diaphragm, thus advancing ignition timing .