The heat riser is a thermostatically or vacuum operated valve in the exhaust manifold. Not all engines have one. It closes when the engine is warming up, to direct hot exhaust gases to the intake manifold, in order to preheat the incoming fuel/air mixture. It it sticks shut, the result will be frequent stalling during warmup, especially in cold and damp weather. If it sticks open, the result will be a rough idle after the engine is warm. There is only one heat riser on a V8. The heat riser should move freely. If it sticks, apply GM Manifold Heat Control Solvent or something similar (engine cool) to the ends of the shaft. Sometimes rapping the end of the shaft sharply with a hammer (engine hot) will break it loose. If this fails, components must be removed for further repairs.