If you have a dwell meter ($35.00) you can get the air fuel (AF) ratio pretty close.

1. Find the diagnostic plug. On the left rear fire wall. Attach the ground from the well meter to pin # 2 on the plug. Attach the other wire from the dwell meter to pin # 3 on the diagnostic plug. Set the meter on Dwell.

2. Crank the engine. Let it get warm. This is so that the oxygen sensor is sending a good signal to the Lambda Control unit.

When the engine is cold the computer sends a fixed signal to the frequency valve. You can see this fixed signal on the dwell meter. The read out on the dwell meter will be 60% duty cycle or about 54 on the four cylinder scale. When the engine gets hot the computer starts looking at the oxygen sensor and adjusting the frequency valve to adjust the exhaust level. If the dwell meter needle swings high (60% to 70% or 54 to 63 on the four cylinder scale) on the scale when the engine is warm the mixture is set to lean. Turn the AF screw on the distributer clockwise just a little. Remove the Allen wrench from the AF screw and let the system settle and look at the dwell meter reading. If the needle swings low (29% to 37% or 26 to 33 on the four cylinder scale) the mixture is set too rich , turn the screw counterclockwise to lean the mixture.

This is explained much better in the book "Bosch Fuel Injection & Engine Management" by Charles O. Probst. Look on pages 32 and 33 for a better explanation.

Thanks

Mark

Troubleshooting Cars with Electronic Carburetors

Reproduced by permission of Bohdan Bodnar

This is a description of the procedure I've used to diagnose air/fuel mixture problems in computer controlled carbureted engines; the outlined procedure can also be used to set the idle air/fuel mixture without resorting to infrared exhaust gas analysis. The procedure is based on the General Motors "System Performance Test" which was developed for diagnosing problems in GM's carbureted engines. This procedure will not work with carburetors which use a stepping motor to control the a/f mixture.

Theory

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 ai r flow. In Fords, only the incoming air is directly varied. In Chryslers, only the incoming fuel flow is directly varied. The valve has a two wires electrical connector. One 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.

DIAGNOSIS AND SETTING IDLE A/F MIXTURE

Monitoring the MC solenoid's average duty requires (for most people) the use of high impedance dwell meter. A low impedance dwell meter may be used unless it affects engine operation; my recommendation is to not use a low impedance dwell meter (that is, stay away from self-powered dwell meters). Following the GM procedure, set the dwell meter to the six cylinders scale REGARDLESS of the number of cylinders in the engine. At this setting, 30 degrees will correspond to a 50% duty cycle, 60 to a 100% duty cycle, and 0 to a 0% duty cycle. Run the engine until closed loop operation is present; this will be indicated by a varying dwell (see footnote 1 for deviations from this procedure). Once the engine is hot, not the average dwell -- the reading should vary equally above 30 degrees and equally below 30 degrees. The following is a brief trouble listing:

1.DWELL NOT VARYING: system is operating in open loop.

2.DWELL STUCK AT 10 DEGREES OR LOWER: full rich command is present; the computer is compensating for WHAT APPEARS TO BE a massive fuel flow reduction (check for dirt in carburetor, air injection system stuck in upstream position, vacuum leaks, improper a/f mixture setting...).

3.DWELL STUCK AT 50 DEGREES OR HIGHER: full lean command is present (check for float stuck low, valve seat damage, oxygen sensor's sense lead shorted to battery voltage, etc.);

4.DWELL OSCILLATING, BUT AVERAGE READING IS BELOW 30 DEGREES: average rich command is present (check for vacuum leaks, dirt in carburetor's jets, improperly set a/f mixture...)

5.DWELL OSCILLATING, BUT AVERAGE READING IS ABOVE 30 DEGREES: average lean command is present. Check for incorrectly set a/f mixture, float stuck low, valve seat damage, clogged air filter, etc...).

Based on the above descriptions, it should be fairly clear on how to set the idle a/f mixture: merely set the mixture so that the average dwell is 30 degrees. Now, suppose the system's dwell is not varying, but the sensors are working properly, the upper radiator hose is hot...

Several cars with small engines have the oxygen sensor mounted fairly far away from the engines. Indeed, during idle conditions, the sensor may cool off to the point that it will not operate (I had this experience in a 1986 Mustang with 2.3 liters engine and EEC-IV system). My recommendation is that all electrical accessories be turned off (so as to provide a minimal load on the engine) and use the idle stop screw on the carburetor to gradually increase the idle rpm until the sensor begins oscillating. Ensuring a negligible load on the engine guarantees that the carburetor will be operating mostly on its idle circuit. Now, set the a/f mixture so that the average dwell is 30 degrees. On the Mustang, this was done at about 1500 rpm.

Note that the a/f mixture setting procedure assumes that NO fuel delivery problems (vacuum leaks, clogged carburetor, etc.) are present.

FOOTNOTES

[1] In some engines (e.g., GM cars with the "min-T" system -- Chevette) the a/f mixture is varied REGARDLESS of whether the engine is in closed loop operation or not. Consider setting the a/f mixture or diagnosing at a slightly increased rpm.