Every 30,000 miles (48,000 km) the heat riser valve should be checked for free operation and lubricated with penetrating oil.
See Figure 1
The valve is located on the exhaust manifold near the point that the exhaust pipe is attached to the manifold. Try to turn the counterweight manually. If it is stuck, tap the end of the shaft a few times with a hammer. Apply penetrating oil to the shaft ends and work back-and-forth a few times. If the valve is still stuck and cannot be loosened with oil or heat (use a propane torch), it will have to be replaced.