- If you're looking for the musical instrument, you're looking in the wrong wiki. Try wikipedia instead.
A banjo is a special fitting that connects the fuel line to the carburetor. Fuel flows from the gas tank through the petcock into the fuel line and fuel filter (you do have a fuel filter, don't you?) and into the banjo fitting or banjo bolt. Fuel passes through the banjo fitting into the carburetor.
It is important to make sure you clean the banjo fitting when you clean the carburetor. Your carburetor can't mix any fuel with air if all the fuel is getting stopped by a clogged up banjo.