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. Q6. Q7. Q8. Name the seven basic carburetor systems? What system maintains a steady working supply of fuel to a constant level in the carburetor? What device acts as a damper to keep the throttle from closing too quickly when the accelerator pedal is suddenly released? 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: 4-32