Wolfgang, the Shop Manual (p. 6-4) has a drawing of the Heat Riser Operation [chevy.oldcarmanualproject.com]. When the flapper valve is in the closed position, routing exhasut gases up under the intake manifold, the counter weight is horizontal. As the spring heats, the counter weight drops toward the engine, going more vertical.
Please see the attached image. The weight will look like this when it is cold. There will be spring pressure against it at this point. The weight rotates counter-clockwise (as the arrow shows) when the engine is at normal running temperature.