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57 Diagnosis And Setting Idle A/F Mixture




Description

This article is from the Chrysler-Plymouth-Dodge FAQ, by Dr. David Zatz with numerous contributions by others.

57 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; 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 UP: 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, 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, 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. During idle conditions, the sensor
may cool off to the point that it will not operate.
Turn off all electrical accessories (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.

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 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.

 

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