Back in the dark ages when I was growing up around the family shop, freeing up the heat riser valve was part of every tune-up. It's pretty important for proper warm-up, routing exhaust gas under the carburetor before the engine reaches normal operating temperature. One stuck in the closed position heats the mixture all the time, resulting in a too-lean mixture during operation at normal running temperature. I don't know of a source for replacement springs. A bimetal spring from a junked automatic choke might work, they operate on the same principle.
Jerry
