Engine fuel system consists of fuel tank, fuel lines, fuel pump, fuel filter, carburetor or fuel injection unit, air cleaner, air filter, and intake manifold.  Fuel is drawn from the tank through the fuel lines by the fuel pump, and is forced through the fuel filter and to the carburetor or fuel injection unit.  At the same time, air is sucked through the air filter by engine vacuum.  The air and fuel are  mixed together in the intake manifold and then distributed to the cylinders.  The ideal air-fuel ratio is 14.7:1 by weight.