Air/Fuel Mixture: Adjustments

1983 - 84 4-140 & 1983 - 87 4-122 Engines
The idle mixture screw is protected by a cap which prevents tampering during normal tune up procedures. If it is evident that idle mixture requires adjustment, remove the cap as outlined under ``Tamper-Resistant Cap Removal'' in the appropriate carburetor section.
1. Apply parking brake and chock wheels. Disconnect automatic brake release, on vehicles so equipped, and plug vacuum connection.
2. Connect suitable tachometer.
3. Disconnect flexible fresh air tube from air cleaner duct or adapter. Using propane enrichment tool T75L-9600-A or equivalent, insert tool hose about 3/4 of way into duct. Secure hose with tape if necessary.
4. Disconnect and plug hose at air bypass valve.
5. Disconnect and plug hose between purge valve and spacer at purge valve. All vacuum signal hoses must remain connected to air cleaner assembly when assembly is relocated to perform carburetor adjustments. Air cleaner assembly must be in place when measuring engine speeds. Prolonged engine idling can result in excessive underbody temperatures due to catalyst overheating.
6. Check that engine timing is correct.
7. On 1985 - 87 models, ensure that curb idle speed is being controlled to specifications, then electrically disconnect idle speed control (ISC) motor from wiring harness.
8. On all models, ensure A/C-heat selector is off and the headlamps are off.
9. Remove PCV valve from grommet and disconnect crankcase closure hose at air cleaner end. This will allow PCV valve and closure hose to vent into engine compartment during idle mixture check.
10. With transmission in Neutral, run engine at about 2500 RPM for 15 seconds before each mixture check. If check is not completed within 60 seconds, repeat this clean-out procedure.
11. With engine idling at normal operating temperature, place transmission in Neutral.
12. Gradually open propane tool valve and observe engine speed gain on tachometer. Note amount of speed gain after engine speed reaches maximum and begins to drop off. (Keep propane cartridge in vertical position.) If engine speed does not drop off, check propane cartridge gas supply. Repeat steps 10, 11, and 12 with new propane cartridge gas supply.
13. If measured speed gain is within 20 - 30 RPM for 1983 vehicles and 10 - 40 RPM for 1984 - 87 vehicles, proceed to step 18. If speed is not within specifications, remove cap as outlined under "Tamper-Resistant Cap Removal" in appropriate carburetor section.
14. If measured speed gain is greater than specified speed gain, turn mixture screw counterclockwise (rich) and repeat steps 10, 11, and 12 until measured speed rise meets reset specification of 30 RPM for 1983 vehicles and 20 RPM for 1984 - 87 vehicles. After final adjustment, proceed to step 17.
15. If measured speed gain is lower than specified speed gain, turn mixture screw clockwise (lean) and repeat steps 10, 11, and 12 until measured speed rise meets reset specifications in step 14. After final adjustment, proceed to step 17.
16. If there is no speed gain and the specified speed gain is 0 RPM, perform the following speed drop test. Speed drop test information is specified with artificial enrichment requirements whenever minimum speed gain specification is 0 RPM.
a. Turn mixture screw 1/4 turn clockwise (lean) while observing drop in engine speed on tachometer.
b. If measured speed drop is greater than 0 RPM and no greater than 30 RPM on 1983--84 vehicles, 40 RPM on 1985 - 87 vehicles, proceed to step 17.
c. If measured speed drop is greater than 30 RPM on 1983 - 84 vehicles, 40 RPM on 1985 - 87 vehicles, turn mixture screw back to original position before adjustment. Proceed to step 17.
d. If measured speed drop is still 0 RPM, repeat steps a, b, and c.
17. Re-install tamper resistant plug.
18. Re-install PCV valve in grommet and reconnect air bypass valve, purge valve and crankcase closure hose.
19. On 1985 - 87 models, reconnect ISC motor to wiring harness.
20. Remove test equipment, reconnect system components, and install air cleaner.