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Gasoline will pick up some heat around the fuel pump.  Some of the heat may come from radiation heat transfer due the gas line's close proximity to the exhaust system and some convective heat transfer may come from hot air blowing across the gas line from the radiator.  It is much easier and more effective to keep heat out of the fuel by insulating the fuel line than it is to keep the heat inside the exhaust system with thermal wraps or ceramic coatings.  The insulation also serves to block thermal radiation from reaching the gas line.

A significant amount of heat entering the fuel will come from conductive heat transfer from the mechanical fuel pump.  The mechanical fuel pump is bolted to the engine block, which will be around the temperature of the water inside the block.  When the fuel flow slows (eg, idling in heavy traffic) or stops (when refueling), less heat will be carried away by the fuel through the fuel pump and the body of the fuel pump will increase and eventually approach the engine block's temperature.  Because gasoline is composed of a variety of components with their own volatities, each component will start to boil off as the fuel pump's temperature reaches its respective boiling point.  The boiling fuel could overwhelm the carburetor's float(s) and cause over-rich operation or flooding.

The following 9:57 minute video shows how cooling the fuel pump restores its fluid flow:

 

A spacer would reduce heat transfer to the fuel pump but could also change the geometry of lever mechanism.

V8 Fuel Pump Spacer SBC Fuel Pump Spacer
Canton Racing PN 85-010
Canton Racing PN 85-000