Manifold Heat Control Valve - stuck close

Having just had to replace my exhaust manifold for this reason, I feel I can answer you pretty well. I'm sure someone will argue my answer, but just think about this! I believe your heat riser valve is operating just fine. When cold, the flap inside the manifold is in the vertical open position. This allows heat from the exhaust to be diverted both out / down into the exhaust pipe, while allowing heat to travel up to the base of the intake manifold warming the carb in the process. As the engine warms, and the spring expands, the flap inside the manifold rotates horizontally into the closed position directing exhaust down the exhaust pipe. From actual experience, on a 95° day, if stuck open the hot exhaust blasting the bottom of the intake will actually boil the gas right out of the carb. Like a Holley 1920 isn't enough of a hazard, this is a great way to burn up a car. Guys who say just cut the flap out must live in cooler climates because I really don't see how it could run properly. I took the new replacement manifold and put it on the bench. Normal temp, valve open. Apply torch, spring expands and valve closes. Cool spring with air, spring shrinks, valve opens. I think one big confusion is that this operates exactly in the reverse of those in the V8 manifolds. The difference being that the carb is not sitting less that a 1/4" from exhaust heat.
I hope that this is helpful.