To further aid cold start driveability during engine warm-up, most 1975 and later engines use a 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.