Heater valve must be installed in such a way that when it is cold the spring action has the valve shut. In other words, it is directing the exhaust towards the bottom of the carburetor. As the engine heats up the bi-metallic spring will get SMALLER and open the valve.
RESTORATION TIP Valve must be free and clear and bushings not leaking. If leaking, manifold should be re-bushed with a new shaft or you will hear a tic-tic-tic sound when the engine is running.
6 cylinder in photo below.