See Figures 1 and 2
Every 30,000 miles (48,000 km), the heat riser valve should be checked for free operation and then lubricated with penetrating oil. The valve is located on the exhaust manifold near the exhaust pipe attachment point. Try to turn the valve counterweight by hand. If it's stuck, tap the end of the shaft a few times with a hammer. Apply penetrating oil to the shaft ends, then work the valve back and forth a few times. If the valve is still stuck and can't be loosened with oil and/or heat, it will have to be replaced.