Choke Heater Tube
The choke housing on the carburetor has a restricted journal to vacuum, which draws air in from the fiber-wrapped heat tube. The tube comes from a fitting on the exhaust manifold connected to a pass-through tube that collects heat from the exhaust. The pass-through is fed with fresh clean air by the tube coming from the top of the carb.
http://forum.garysgaragemahal.com/1984-F-150-302-Windsor-2WD-Old-Red-tp8076p25268.html
