The purpose of the hot air control system is to reduce HC emissions and improve engine performance during warm-up. This is accomplished by deflecting either cool outside air or warm engine-heated air into the carburetor, depending upon engine operating conditions.
On 1970–76 models a two position control valve, mounted in the air cleaner snorkel, allows either underhood air or preheated air to flow into the air cleaner. The valve is controlled by a lever marked HOT and COLD, or WINTER and SUMMER.
When the ambient temperature is below 59°F (15°C), set the lever to the HOT or WINTER position. If the temperature is above 59°F (15°C), move the lever to the COLD or SUMMER position.
On the 1977–84 system works automatically be means of a temperature sensor and vacuum motor. The temperature sensor detects the inlet air temperature and controls the flow of vacuum to the vacuum motor. Together they regulate the air control valve, mounted in the air horn. The possible combinations of inlet (underhood) air temperatures and vacuum readings, and the resulting valve operation are shown on the Operation of the Air Control Valve chart.