4. MAIN METERING SYSTEM. With the engine warmed up and running at 2,000 rpm, slowly cover part of the air horn with a piece of stiff cardboard The engine should speed up slightly, since this action causes a normal operating main metering system to discharge more fuel.

WARNING

Do NOT use your hand to cover the air horn when performing this test.

Q5. Name the seven basic carburetor systems?

Q6. What system maintains a steady working supply of fuel to a constant level in the carburetor?

Q7. What device acts as a damper to keep the throttle from closing too quickly when the accelerator pedal is suddenly released?

Q8. What sensor in a computerized carburetor system measures intake vacuum and engine load?

GASOLINE FUEL INJECTION SYSTEMS

LEARNING OBJECTIVE: Identify and describe the different gasoline fuel injection systems.

A modern gasoline injection system uses pressure from an electric fuel pump to spray fuel into the engine intake manifold. Like a carburetor, it must provide the engine with the correct air-fuel mixture for specific operating conditions. Unlike a carburetor, however, PRESSURE, not engine vacuum, is used to feed fuel into the engine. This makes the gasoline injection system very efficient.

A gasoline injection system has several possible advantages over a carburetor type of fuel system. Some advantages are as follows:

Improved atomization. Fuel is forced into the intake manifold under pressure that helps break fuel droplets into a fine mist.

Better fuel distribution. Equal flow of fuel vapors into each cylinder.

Smoother idle. Lean fuel mixture can be used without rough idle because of better fuel distribution and low-speed atomization.

Lower emissions. Lean efficient air-fuel mixture reduces exhaust pollution.

Better cold weather drivability. Injection provides better control of mixture enrichment than a carburetor.

Increased engine power. Precise metering of fuel to each cylinder and increased air flow can result in more horsepower output.

Fewer parts. Simpler, late model, electronic fuel injection system have fewer parts than modern computer-controlled carburetors.

There are many types of gasoline injection systems. Before studying the most common ones, you should have a basic knowledge of the different classifications. Systems are classified either single- or multi-point injection and indirect or direct injection.

The point or location of fuel injection is one way to classify a gasoline injection system. A single-point injection system, also call throttle body injection (TBI), has the injector nozzles in a throttle body assembly on top of the engine. Fuel is sprayed into the top center of the intake manifold

A multi-point injection system, also called port injection, has an injector in the port (air-fuel passage) going to each cylinder. Gasoline is sprayed into each intake port and toward each intake valve. Thereby, the term multi-point (more than one location) fuel injection is used.

An indirect injection system sprays fuel into the engine intake manifold. Most gasoline injection systems are of this type. Direct injection forces fuel into the engine combustion chambers. Diesel injection systems are direct type.

There are three basic configurations of gasoline fuel injection - timed, continuous, and throttle body.

TIMED FUEL INJECTION SYSTEM

Timed fuel injection systems for gasoline engines inject a measured amount of fuel in timed bursts that are synchronized to the intake strokes of the engine. 'limed injection is the most precise form of fuel injection but is also the most complex. There are two basic forms of timed fuel injection-mechanical and electronic.

The basic operation of a mechanical-timed injection system (fig. 4-43) is as follows: