Vehicle Testing Procedures
6-9
Lean-Misfire Adjustment Test
1. Observe the CO reading.
— If greater than zero, proceed to step 2.
— If not greater than zero, turn the mixture screw(s) to
enrich the mixture. If there are two adjustment
screws, adjust both screws an equal number of turns.
2. Lean the mixture with the adjustment screw(s) 1/4 turn at a
time until HC begins to increase. Pause between
adjustments and allow the readings to stabilize.
3. Back the screw(s) out 1/4 turn.
4. Compare the CO and HC readings with the specifications
of the engine manufacturer and readjust the mixture as
required to obtain the correct readings.
Lean-Drop Adjustment Test
1. Turn the mixture screw(s) to obtain the highest RPM. If
there are two adjustment screws, adjust each screw for a
10 RPM drop (20 RPM total) and back each one out 1/4
turn.
2. Lean the mixture with the adjustment screw until a 20 RPM
drop is obtained, then adjust 1/4 turn richer. If there are
two mixture screws, adjust both screws for a 10 RPM drop
and back off 1/4 turn.
3. Compare the CO readings with the specifications of the
engine manufacturer and readjust the mixture as required
to obtain the correct readings.
Interpretation
If the required exhaust gas readings cannot be obtained by
following manufacturer recommended procedures, further
diagnostic testing is required. Possible causes of problems
include:
•
Faulty ignition or compression,
•
Vacuum leaks,
•
PCV system malfunctions, and
•
Carburetor related problems.
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