See Figure 1
Some engines are equipped with a valve installed in the exhaust manifold. The valve is operated by a bi-metallic spring which hold the valve shut while cold and gradually opens the valve as the manifold heats up. The purpose of this valve is to aid in cold weather operation by decreasing warm-up time. Service to the valve and spring is limited to checking the freedom of operation and regularly lubricating the spring and valve shaft with spray lubricant especially made for heat risers.