See Figures 1 and 2
Some models are equipped with exhaust control valves, or heat risers near the headpipe in the exhaust manifold. These valves aid initial warm-up in cold weather, by restricting exhaust gas flow slightly. The heat generated by this restriction is transferred to the intake manifold where it results in improved fuel vaporization.
The operation of the exhaust control valve should be checked every 6 months or 6000 miles (9661 km). Make sure that the thermostatic spring is hooked on the stop pin and that the tension holds the valve shut. Rotate the counter weight by hand and make sure that it moves freely through about 90º degrees of rotation. A valve which is operating properly will open when light finger pressure is applied (cold engine). Lubricate the shaft bushing with a mixture of penetrating oil and graphite. Operate the valve manually a few times to work in the lubricant.