318 exhaust manifold has a buterfly flap??

You find these on the passenger side manifold- they are a diverter of exhaust heat into the intake manifold crossover passage (original equipment iron single plane 2 barrel) to help warm up and atomize the fuel/air mix "better" during cold startup. Think of it as a Jurassic attempt at improving the emissions of your engine for meeting EPA standards of way back.

The disc the valve is attached to is weighted and the spring is bi-metallic, so that as it heats up it opens the valve inside the manifold and no longer restricts your exhaust. If you accelerate the motor (cold), the valve is supposed to open from the increased exhaust velocity, then close again until it heats up. Most of us take it out and weld closed the ends to seal the resulting leaks- or replace them with something better. Not too interested in heating the fuel air these days- colder charge is better. . .and not especially interested in whether the car meets emissions standards upon startup either.