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.
Computerized carburetor systems vary.
For exact detail on a particular system, refer to
the manufacturers service manual, which will
explain how the specific system functions.
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
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
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
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
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
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.