Hy GrandpasTruck, the heat riser Hotrod Lincoln is talking about is the valve in the exhaust manifold which controls the flow of exhaust out of the exhaust manifold. On Chevies the valve is controlled by a thermostatic spring, on GMCs the valve can be adjusted and locked into position. Many of the valves are siezed in position, or the springs are broken, or the stud that the spring pushes against is rusted off. Better do some investigating, hope that helps.
