That flap valve is known as a "heat riser"- - - -During engine warmup, it directs exhaust gas against the bottom of the intake manifold to help keep the gasoline vapors from condensing back into a liquid and prevent uneven fuel/air mixtures in the cylinders. Once the engine coolant reaches normal operating temperatures, manifold heat is not needed, so a coiled bimetal spring on the shaft relaxes and the counterweight causes the flap valve to open. Using a torch and some penetrating oil, and tapping the shaft fore and aft repeatedly is the best way to free it up without damaging anything. Be patient- - - -it's taken me a couple of hours and repeated heat/tap/repeat cycles at times to free up a heat riser! This spring works well as a replacement, and it's a fraction of the cost of an OEM stovebolt item:
www.ebay.com/itm/182196172425?Jerry
