High Altitude Compensation System: Description

As altitude increases, air pressure decreases and the air/fuel mixture becomes richer. The High Altitude Compensation (HAC) system supplies additional air to the primary low and high speed circuits on all models, and secondary high speed circuits on some models. This helps to reduce emissions and improve driveability. Ignition timing is also advanced for better driveability.

By changing the amount of air supplied to carburetor circuits, this system is designed to be functional at all altitudes. The system includes a high altitude compensation valve, a vacuum advance sub-diaphragm, and a check valve.