Trap Door Inside Exaust Manifold

weldedrail said

So what controls this? Is there a linkage or something missing that is supposed to connect to the carb or something?

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Like green1 said. The spring on the back side of the large round weight is bimetal. That means when it gets hot it expands. As it expands, it causes the valve to open up and divert exhaust more through the exhaust pipe.