Pages 4-93 & 4-94
1986
Carburetors, Fuel Charging Assemblies and Throttle Bodies 4-93
Idle Mixture Setting Procedures
IDLE FUEL MIXTURE-PROPANE ENRICHMENT METHOD
4.9L TRUCK UNDER 8500 GVW w/YFA-IV FB
NOTE: This procedure is for the 4.9L engine only. It is not for trucks over 8600 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. Disconnect the automatic brake release (if so equipped), and plug the vacuum connection.
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 adapter. 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. 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. Place A/C-heat selector in the Off position and verify that all accessories (lamps, defrost, etc.) are Off.
1986
4-94 Carburetors, Fuel Charging Assemblies and Throttle Bodies
Idle Mixture Setting Procedures
Figure 1 - In-Line Base Timing Connector:
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).
C. Verify that the idle speed control motor and the single wire connector near the distributor (sometimes called the In-Line Base timing connector, Figure 1) are connected electrically. 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.
D. Return engine to idle and disconnect idle speed control motor from wiring harness. Disconnect single wire connector near the distributor (Fig. 1). With the engine idling at normal operating temperature, place the transmission selector in the position specified for the idle fuel mixture check. 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 C and D with a new propane cartridge gas supply.
E. Compare measured speed gain to specified speed gain.*
1. If the measured speed gain is within specification, proceed to Step G.
NOTE: The carburetor assembly is equipped with a tamper-resistant idle adjustment feature, the carburetor must be removed from the vehicle. Refer to Group 24, Powertrain Shop manual 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 C and D until the measured speed rise meets the reset rpm specification,* After final adjustment, proceed to Step F.
3. If the measured speed gain is lower than the speed gain specification:*
Turn the mixture screw clockwise (lean) and simultaneously repeat Steps C and D until the measured speed rise meets the reset rpm specification.* After final adjustment, proceed to Step F.
F. The carburetor fuel air adjustment must have the tamper-resistant feature reinstalled. Refer to appropriate carburetor assembly instructions in Shop Manual, Group 24.
G. Reinstall PCV valve in grommet. Reconnect idle speed control motor and crankcase closure system, and the single wire connector near the distributor.
H. Remove all test equipment. Reconnect all system components, reinstall air cleaner, and tighten the air cleaner wing nut(s) to specification, *Specified speed gain is 10 to 70 rpm, specified reset is 30 rpm.