I recently found the heat riser flapper on my truck frozen in the shut position. And the spring was missing. According to the shop manual, when it is stuck in the "heat on" position, the result is poor performance, overheating and detonation. If it is stuck in the "heat off" position, the result is poor performance, particularly during warm up and low speeds. Assuming you have the same balance weight system that I do, the proper spring tension is one half turn of the spring from its relaxed position. You can test it with a decent heat gun but be sure the manifold is standing in the proper orientation if you are testing it off the truck or else the balance weight fouls up the test.
