BBD Carburetor - Idle Mixture Adjustment TSB
David Harmon
as provided to me by the cool parts dude Steve at my local Jeep
dealer. Capitalization and emphasis are theirs. Typos are mine.
I hope this information will be useful to the group.
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Carburetor - Idle Mixture Adjustment
Models
All 1990 & Prior Domestic Vehicles With 4.2L I6 Engines With Fuel
Feedback System
Subject
Idle Mixture Adjustment Procedure
Index
FUEL SYSTEM
Date
October 16, 1989
No.
14-53-89
P-3738 (C14-11-9)
BODY CODE LEGEND
ALL 1990 & PRIOR DOMESTIC VEHICLES WITH 4.2L I6 ENGINES WITH FUEL
FEEDBACK SYSTEM
IDLE MIXTURE ADJUSTMENT FOR 4.2L I6 ENGINES
This Technical Service Bulletin is to advise dealer technicians of
the procedure for carburetor idle mixture adjustment on all vehicles
produced with the 4/2L I6 engine equipped with the fuel feedback
system.
ADJUSTMENT PROCEEDURE
NOTE: THE IDLE MIXTURE SHOULD ONLY BE PERFORMED IF THE ADJUSTMENT
SCREWS WERE REMOVED DURING CARBURETOR OVERHAUL FOR CLEANING
PURPOSES
WARNING: USE EXTREME CAUTION WHEN THE ENGINE IS OPERATING. DO NOT
STAND IN A DIRECT LINE WITH THE FAN. DO NOT PUT YOUR HANDS
NEAR THE PULLEYS, BELTS, OR FAN. DO NOT WEAR LOOSE CLOTHING.
1. Set the parking brake firmly. Position the gear selector in
NEUTRAL for manual transmissions and PARK for automatic
transmissions.
2. Connect a tachometer with an expanded scale of 400-800 or 0-1000
RPM. Inspect periodically to ensure that accuracy is within two
percent. Start the engine and warm it to the normal operating
temperature.
3. Stop the engine. Restart the engine and disconnect the stepper
motor connector within 15 seconds of engine restart. Stepper motor
will be in the initialized/midpoint position. PLEASE NOTE THAT THIS
STEP SHOULD BE INSERTED INTO THE "IDLE MIXTURE ADJUSTMENT" PROCEDURE
AS WRITTEN IN THE SERVICE MANUAL.
4. Position the gear selector in NEUTRAL for manual transmissions
and DRIVE for automatic transmissions.
5. Adjust the idle speed as described in the "Idle Speed
Adjustment" procedure of the Service Manual. Adjust the Curb Idle
Speed "Set To" RPM listed in Specifications.
6. Adjust the mixture screw(s) leaner (clockwise) until a
perceptible loss in RPM is noted.
7. Turn the mixture screw(s) richer (counterclockwise) until the
highest RPM indication is obtained. Do not turn the screws any
further than the point at which the highest RPM is first obtained.
This point is referred to as LEAN BEST IDLE. The engine speed will
increase above the curb idle speed by an amount that corresponds to
approximately the IDLE DROP specification to be obtained in the next
step.
8. As a final adjustment, turn the mixture screws clockwise
(leaner) to obtain the specified drop in engine idle RPM. Turn both
of the idle mixture screws in small equal amounts until the
specified IDLE DROP is achieved. Refer to Mixture Adjustment in the
appropriate Service Manual.
9. Reconnect the stepper motor connector.
10. Install RTV in the idle mixture screw plug holes.
If the final engine RPM differs more than +/- 30 RPM from the
original curb idle RPM, adjust the curb idle speed to the specified
RPM.
POLICY: Reimbursable within the provisions of the warranty.
TIME ALLOWANCE
Labor Operation No. 14-30-01-91 . . . . . . . .0.2 Hrs.
FAILURE CODE: XX - Service Adjustments
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David Harmon
Manual are:
Curb Idle SET TO = 680 RPM +/- 50
IDLE DROP = 50 RPM.
The specifications for your year may vary, but probably not.
Always try to adjust both mixture screws by the same amount,
starting from all the way in.
Mike Romain
It is nice to know it 'can' be done without using a tailpipe sniffer or
taking guesses to get the stepper centered...
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's