Pages 4-69 & 4-70
1987
Carburetors, Fuel Charging Assemblies and Throttle Bodies 4-69
Idle Mixture Setting Procedures
IDLE FUEL MIXTURE-PROPANE ENRICHMENT METHOD
2.0L TRUCK UNDER 8500 GVW w/Aisan Carburetor
NOTE: This procedure is for 2.0L Truck engines only, it is not for trucks over 8500 GVW. Propane specifications are not listed for these applications. If CO and HC are not within specification, perform appropriate electrical diagnostic procedure, Section 15.
I. Vehicle Preparation
A. Apply the parking brake and block the wheels.
B. Connect an analog tachometer, Rotunda 059-000010 or an equivalent analog tachometer.
C. Disconnect the flexible fresh air tube from the air cleaner duct or adaptor. Using propane enrichment Tool T75L-9600-A or equivalent, insert the tool hose approximately 3/4 of the way into the duct or fresh air tube. If necessary, secure the hose with tape.
D. Disconnect and plug the hose at the air bypass valve.
E. Disconnect and plug the hose between the purge valve and the spacer at the purge valve.
F. When the above Steps have been completed, proceed to Section II.
II. Propane Enrichment Method (Except as Noted)
NOTE: Leave all vacuum signal hoses attached to the air cleaner assembly when relocating the assembly to perform carburetor adjustments. The air cleaner assembly must be in place, however, when measuring engine speeds.
CAUTION: Prolonged engine idling can result in catalyst overheating and excessive underbody temperatures.
A. Verify engine timing is set to specification-reset if necessary, Section 15.
B. Verify that the curb idle speed is being controlled to specification. Then, electrically disconnect idle speed control (ISC) motor from wiring harness.
C. Place A/C-heat selector in the Off position and verify that lamps are Off.
NOTE: Remove PCV valve from grommet, locate the crankcase closure hose and disconnect at the air cleaner end (allowing the PCV valve and closure hose to vent to underhood air during idle fuel mixture check).
D. With the transmission in Neutral, run the engine at approximately 2500 rpm for 15 seconds before each mixture check. If check is not completed within 60 seconds, repeat this cleanout Step.
1987
4-70 Carburetors, Fuel Charging Assemblies and Throttle Bodies
Idle Mixture Setting Procedures
E. With the engine idling at normal operating temperature gradually open the propane tool valve and watch for engine speed gain, on the tachometer. When the engine speed reaches a maximum and then begins to drop off, note the amount of speed gain. (Propane cartridge must be in vertical position).
NOTE: If the engine speed will not drop off check the propane cartridge gas supply. If necessary repeat Steps D & E with a new propane cartridge gas supply.
F. Compare measured speed gain to specified speed gain.*
1. If the measured speed gain is within specification, proceed to Step H.
NOTE: The carburetor assembly is equipped with a tamper-resistant idle adjustment feature, the carburetor must be removed from the vehicle. Refer to Shop Manual, Group 24 for removal of the above feature.
NOTE: If it is determined that idle fuel mixture adjustment will be necessary, adjust the mixture according to the reset rpm specification.
2. If the measured speed gain is higher than the speed gain specification:*
Turn the mixture screw counterclockwise (rich) and simultaneously repeat Steps D and E until the measured speed rise meets the reset rpm specification.* After final adjustment, proceed to Step G.
3. If the measured speed gain is lower than the speed gain specification:*
Turn the mixture screw clockwise (lean) and simultaneously repeat Steps D and E until the measured speed rise meets the reset rpm specification.* After final adjustment, proceed to Step G.
4. If there is no rpm rise and the minimum speed gain specification is "0" rpm, perform the following speed drop test:
NOTE: Speed drop test information is specified with the artificial enrichment requirements whenever the minimum speed gain specification is "0" rpm.*
a. While watching the tachometer, adjust the mixture screw 1/4 turn clockwise (lean). Note the drop in engine speed.
b. If the measured speed drop is greater than "0" and less than or equal to 40 rpm, proceed to Step G.
c. If the measured speed drop is more than 40 rpm, return the mixture screw to the position before adjustment. Proceed to Step G.
d. If the measured speed drop is still "0" rpm repeat steps a, b, and c.
G. The carburetor fuel air adjustment must have the tamper-resistant feature reinstalled, see Section III.
*Refer to the Special Specifications Issue TSB Publications.