See Figure 1
To further aid cold start driveability during engine warm-up, most 1975 and later engines use a Vacuum Operated Heat Valve (VOHV) located between the exhaust manifold and the exhaust inlet (header) pipe.
When the engine is first started, the valve is closed, blocking exhaust gases from exiting from one bank of cylinders. These gases are then diverted back through the intake manifold crossover passage under the carburetor. The result is quick heat to the carburetor and choke.
The VOHV is controlled by a ported vacuum switch which uses manifold vacuum to keep the vacuum motor on the valve closed until the coolant reaches a predetermined warm-up valve. When the engine is warmed-up, the PVS shuts off vacuum to the VOHV, and a strong return spring opens the VOHV butterfly.
Testing the vacuum operated heat riser valve is a matter of making sure it opens and closes freely. You can move it by hand to see if it works, on a cool engine. When the engine is cold, the valve should be closed, and disconnecting the hose should allow it to open (engine idling). On a cold engine, there should be vacuum at the vacuum actuator. On a warm engine the vacuum should be shut off.