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The only direct mention of the hear riser I can find in the 54 Truck Service manual is page 6-57, and that's to turn off the auto-choke. Which was optional to begin with. Model BC carbs were only equipped on trucks with automatic transmissions.
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Here is the explanation of the
thermostatically operated Heat Control Valve (and Manifold Heat Riser) in the 1954 Chevrolet Truck Shop Manual
Here is the
Manifold Heat Valve adjustment instruction in the 1954 Chevrolet Truck Shop Manual
