This article is from the Chrysler-Plymouth-Dodge FAQ, by Dr. David Zatz with numerous contributions by others.
From: Bohdan L Bodnar
This is the procedure I've used to diagnose air/fuel
mixture problems in computer controlled carbureted engines; the
procedure can also be used to set the idle air/fuel mixture
without exhaust gas analysis. The procedure is
based on the General Motors System Performance Test.
The a/f mixture is controlled by a MIXTURE CONTROL SOLENOID (MC
solenoid). This is a valve which operates at a fixed frequency
(typically, 10 Hz) and whose duty cycle (valve's ON time divided by
period) is varied. That is, the valve is pulse width modulated. When
the valve is turned on, the incoming a/f mixture is fully leaned; when
off, fully enrichened. The former is called a "lean command" whereas
the latter is called a "rich command." By varying the duty cycle of the
MC solenoid, the AVERAGE a/f mixture can be varied. In GM products,
this valve directly varies the incoming fuel and air flow. In Chryslers, only
incoming fuel flow is directly varied.
The valve has a two wires electrical connector. On wire is connected to
switched battery voltage whereas the other is connected to a power
transistor in the computer and is a source of switched ground.
During closed-loop operation the following will occur (assume the oxygen
sensor is sensing a lean condition its voltage will be low):
1). The computer gradually decreases the MC solenoid's duty cycle.
2). The exhaust eventually becomes rich enough that the oxygen sensor's
output will swing high (about 1 volt).
3). The computer gradually increases the MC solenoid's duty cycle.
4). The exhaust eventually becomes lean enough that the oxygen sensor's
output will swing low (about 0 volt).
The cycle now repeats. A device for monitoring the solenoid's duty
cycle (such as a dwell meter) will show a constantly varying duty cycle.
The frequency of the oscillations will depend on the how fast the
computer varies the duty cycle and the engine's RPM. An AVERAGE duty
cycle of 50% corresponds to, on the average, NO average a/f correction.
Stated differently, everything is operating correctly. An average duty
cycle of LESS THAN 50% corresponds to, on the average, a rich command
(the computer is compensating for a lean condition). An average duty
cycle GREATER THAN 50% corresponds to, on the average, a lean command.