Small Engine Fuel System

The most common repairs involve the fuel system, which includes the fuel tank, fuel filter, fuel line, fuel pump (on some models) and carburetor. You may have heard people jokingly comment that the fuel in the tank is so low the engine is running "on vapors." Technically, they're right. Gosoline won't burn in its liquid state; it must be converted to a vapor first. The vapors that burn in your small engine are formed from a mixture of fuel (typically gasoline) and air. And you need the right amount of fuel and the right amount of air to maintain whatever engine speed you select. The best way to understand the fuel system is to begin at the tank.

FROM THE FUEL TANK TO THE CARBURETOR

Located the fuel tank on your engine. If you have an older engine, it's probably made of steel or aluminum. Newer tanks are made of plastic and are built into the molded plastic shroud over the engine. Now look for a fuel line, a hose connected to one side of the tank. The fuel lone carries the fuel to the carburetor, a mixing chamber that contains a throttle and (if equipped) a choke, attached to the equipment controls. On most engines, the force of gravity carries the fuel through the fuel line. However, if the fuel tank is mounted low on the engine, gravity may not do the trick. In this case, a fuel pump uses low pressure in the crankcase to pump the fuel. The pump is located between the tank and the carburetor or in the carburetor itself. Some engines eliminate the need for either a fuel line or pump by mounting the carburetor directly on the fuel tank and using a pick-up tube in the tank to draw fuel.

INTO THE CARBURETOR

On most engines, fuel from the fuel line enters the carburetor's fuel bowl, a resevor where a float, similar to the float ball in a toilet tank, regulates the fuel level. From there, a metering device called a jet lets fuel into the emulsion tube inside the pedestal, where fuel and air first mix. (Older models include an adjustable jet; newer models contain a fixed jet.) Fuel travels through the emulsion tube to the main passageway in the carburetor, called the throat or venturi, where further mixing occurs. If your carburetor is a tank-mounted type, fuel from the tank may be supplied directly to the emulsion tube, without the need for a float.

THE ROLE OF THE THROTTLE

At one end of the throat is a throttle plate. The throttle plate is connected to your equipment control lever (often referred to as the throttle) and opens or closes to increase or decrease engine speed. As the throttle plate opens, more air is drawn into the carburetor. Air flow, in turn, determines how much fuel is delivered for combustion. Many carburetors have an idle speed screw to stop the throttle from closing too far at low speed, and in idle mixture screw, which increases or decreases air and fuel flow to prevent a stall.

UNDERSTANDING CARBURETION

In essence, a carburetor is a passageway that drawn in air and fuel and supplies a mixture of the tow to the cylinder. It's the narrowing of the passageway, called the throat or venturi, that causes the carburetor to draw in the two components the engine nees for combustion. Basic physics tells me that air speed will increase at the narrow point and air pressure will drop. Since fluids flow to low-pressure areas, fuel from the bowl or ank is drawn into the throat, mixing with air to form a combustible vapor.