The COMPUTER, also called the electronic control unit (ECU), uses sensor information to operate the mixture control solenoid of the carburetor. The IDLE SPEED ACTUATOR is a tiny electric motor and gear mechanism that allows the computer to change engine idle speed by holding the throttle lever in the desired position. Many of the components and sensors are also used in gasoline fuel injection systems, which we will discuss later in this chapter. In a computer-controlled carburetor, the air-fuel ratio is maintained by cycling the mixture solenoid ON and OFF several times a second. Control signals from the computer are used to meter different amounts of fuel out of the carburetor.   When the computer sends a rich command to the solenoid, the signal voltage to the mixture solenoid is in the OFF position more than it is ON, causing the solenoid to stay open more. During a lean signal the mixture solenoid has more ON time, causing less fuel to pass through the solenoid valve and the mixture becomes leaner. NOTE Computerized carburetor systems vary. For exact detail on a particular system, refer to the manufacturer’s service manual, which will explain how the specific system functions. CARBURETOR  TROUBLES Some of the engine troubles that can usually (but not ALWAYS) be traced to some fault in the carburetor system are as follows: EXCESSIVE  FUEL  CONSUMPTION  can result from a high float level, a leaky float, a sticking metering  rod  or  full  power  piston,  a  sticking  accelerator pump, and/or too rich of an idling mixture. A SLUGGISH ENGINE may be the result of a poorly operating accelerator pump, a low float level, dirty or gummy fuel passages, or a clogged air cleaner. POOR  IDLING,  often  characterized  by  a  stalling of the engine, is usually due to a too rich idle mixture, a defective choke, or an incorrectly adjusted idle speed screw at the throttle plate. FAILURE OF THE ENGINE TO START may be caused by an incorrectly adjusted choke, clogged fuel lines, or air leak into the intake manifold. HARD STARTING OF A WARM ENGINE could be due to a defective or improperly adjusted throttle  link. SLOW ENGINE WARM-UP may indicate a defective choke or defective radiator thermostat. SMOKY BLACK EXHAUST indicates a very rich air-fuel mixture. STALLING OF THE ENGINE AS IT WARMS could be caused by a defective choke or closed choke valve. A BACKFIRING ENGINE may be due to an incorrect, often lean, air-fuel mixture reaching the engine.  In  turn,  this  condition  could  be  caused  by  a clogged fuel line or a fluctuating fuel level. An ENGINE RUNS BUT MISSES, the most likely cause is a vacuum leak at a vacuum hose or the intake manifold. In addition, it could be an improper air-fuel mixture reaching the engine due to clogged or worn carburetor jets or an incorrect fuel level in the float bowl. Several quick checks can be made to see how well the carburetor is working. More accurate analysis requires  test  instruments,  such  as  an  exhaust  gas analyzer and an intake manifold vacuum gauge. The quick checks are as follows: 1. FLOAT LEVEL ADJUSTMENT. With the engine warmed up and running at idle speed, remove the air cleaner. Carefully note the condition of the high- speed  nozzle.  If the nozzle tip is wet or is dripping fuel, the float level is probably too high. This could cause a continuous discharge of fuel from the nozzle, even at idle. 2. IDLE SYSTEM. If the engine does not idle smoothly after it is warmed up, the idle system could be at  fault.  Slowly  open  the  throttle  until  the  engine  is running at about 3,000 rpm. If the speed does not increase evenly and the engine runs roughly through this speed range, the idle or main metering system is probably   defective. 3. ACCELERATOR PUMP SYSTEM. With the air cleaner off and the engine not running, open the throttle suddenly. See if the accelerator pump system discharges a squirt of fuel into the air horn. The flow should continue for a few seconds after the throttle plate reaches the wide, open position. 4-31