GASOLINE FUEL SYSTEM

The three basic parts of a gasoline fuel system are the fuel tank, fuel pump, and carburetor. Fuel is supplied from the fuel tank to the carburetor by either a gravityfeed system or a force-feed system. The gravity-feed system has the fuel tank placed above the carburetor (fig. 1-24). Afloat attached to a valve allows fuel to enter the carburetor at the same rate at which the engine is consuming it. This system maintains a uniform level in the carburetor regardless of the amount of fuel in the tank. The force-feed system (fig. 1-24) is where the fuel tank is located below the carburetor and a fuel pump is required.

Fuel Pump

The fuel pump draws the gasoline through a fuel line from the tank and forces it to the float chamber of the carburetor where it is stopped. Several types of fuel pumps are used; however, the most common type is the mechanical nonpositive fuel pump (fig. 1-25).

Carburetor

The carburetor is basically an air tube that operates by a differential in air pressure. It has an hourglassshaped tube called a throat and the most constricted part

Figure 1-23.-Secondary fuel filter.

Figure 1-24.-Fuel systems.

Figure 1-25.-Mechanical nonpositive pump.

Figure 1-26.-Venturi effect.

Figure 1-27.-Throttle valve.

of the throat is called the venturi (fig. 1-26). A tube called a discharge nozzle is positioned in the venturi and is connected to a reservoir of gasoline called the float bowl.

The downward intake stroke of the piston creates a partial vacuum in the carburetor throat that allows low-pressure air to rush by the fuel nozzle. This forces small drops of fuel to be mixed with the air. Then the fuel and air mixture must pass the throttle valve which is controlled by the operator (fig. 1-27). The throttle valve opens or closes to allow the correct volume of the fuel and air mixture into the engine. The choke valve (fig. 1-28) also controls the supply of fuel to the engine. When you start the engine in cold weather, the choke valve can be partly closed, forming a restriction that causes more fuel and less air to be drawn into the combustion chamber. This results in a richer air to fuel

^{Figure 1-28.-Choke valve operation.}

Figure 1-29.-Diesel fuel system.

mixture in the cylinders for the harder job of starting at low temperature.