Tab- the heat riser valve is in the stock exhaust manifold right below the carb. It it a valve that either sends exhaust out the tailpipe or diverts some up through the intake manifold, around the base of the carb, and back out the exhaust manifold. It helps the mixture be correct when the engine is cold. If it is stuck you experience mixture problems. There is a weight on one side that hangs on the shaft and a coiled spring on the other side.
Tony M-
To check if it is working correctly, start the engine cold and watch the weight. The weight should rotate about 90 degrees as the engine gets hot.
