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Old 10-09-2008, 08:52 PM
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the heat riser is usually mounted on one side of the exhaust, right were the manifold connects to the pipe. On older vehicles, it has a thermostatic spring that will open the valve once it warms up. It's closed cold to help force the exhaust through the passage in the head and into the passage in the intake, to help heat up the mixture and carb. Newer cars may have a vacuum operated one or an electric one.

the door in the air cleaner snorkel is usually controlled by vacuum and that hose goes through a thermal vacuum switch located (usually) in the thermostat housing. The metal tube, or pipe that does down from the snorkel to the exhaust manifold draws warm air into the carb, again helping the mixture and warming the carb. This is expecialy helpful if you experience carb frosting. (not to be confused with chocolate frosting!)
Once the water temp reaches the rating on the TVS, it will change the signal and the door will change position and let in either under hood air or, if you have a snorkel that goes to the core support, fresher outside air.
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