Gasoline Engines Only
The heat riser, or Early Fuel Evaporative System, is a thermostatically operated valve in the exhaust manifold. It closes when the engine is warming up to direct hot exhaust gases to the intake manifold, in order to preheat the incoming air/fuel mixture. It is used on carbureted engines only. If it sticks shut, the result will be frequent stalling during warmup, especially in cold or damp weather. If it sticks open, the result will be a rough idle after the engine is warm.
The heat control valve should be checked for free operation every six months or 6,000 miles. Simply give the counterweight a twirl (engine cold) to make sure that no binding exists. If the valve sticks, apply a heat control solvent to the ends of the shaft. This type of solvent is available in auto parts stores. Sometimes lightly rapping the end of the shaft with a rubber hammer (engine hot) will break it loose. If this fails, the components will have to be removed from the car for repair.
The 1980 and later carbureted engines do not use the heat control valve. Instead, these engines warm the fuel mixture by a coolant passage under the carburetor. No maintenance is required.