The following is the full text of a Jeep Technical Service Bulletin 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.
14-53-89P-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.
- Set the parking brake firmly. Position the gear selector in NEUTRAL for manual transmissions and PARK for automatic transmissions.
- 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.
- 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.
- Position the gear selector in NEUTRAL for manual transmissions and DRIVE for automatic transmissions.
- 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.
- Adjust the mixture screw(s) leaner (clockwise) until a perceptible loss in RPM is noted.
- 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.
- 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.
- Reconnect the stepper motor connector.
- 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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