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Re: exhaust manifold
The totally official reason for this thermostatic spring controlled flapper valve is to provide a slight backpressure on that side of the motor when it is started on a cold day. That allows a small amount of exhaust gasses to go up into the intake manifold on the heat riser section and heat the area under the carb. There are two things that accomplishes (1) to heat the fuel charge as it goes into the cylinders and (2) to heat the choke spring that was mounted in the earlier trucks intake manifold. This assisted in the choke in "Pulling Off" after being set by cold temperatures. Most of us eliminated this thing a long time ago.
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