Idle Mixture Adjustment

The idle mixture screw is protected by a plug which prevents tampering during normal tune up procedures. If it is evident that idle mixture requires adjustment, remove plug as outlined under ``Concealment Plug Removal'' in the appropriate carburetor section.




4-121 2 BBL. ENGINE
Idle mixture adjustment requires artificial propane enrichment. Propane enrichment tool J-26911 or equivalent, is required to adjust idle mixture. Air cleaner must remain in place during adjustments. A/C control, if equipped, must be in off position.
1. Set parking brake, then block drive wheels.
2. Disconnect and plug EGR valve and canister purge hoses.
3. Check and, if necessary, adjust ignition timing.
4. Connect tachometer to engine following manufacturer's instructions, then start engine and allow to reach normal operating temperature.
5. Set curb idle speed to specifications.
6. Disconnect crankcase ventilation tube from air cleaner. Insert hose with rubber stopper from propane valve into opening in air cleaner. Propane cylinder must remain upright during adjustments, and cylinder must be adequately filled to ensure correct propane flow.
7. With engine idling in Drive (auto. transmission) or Neutral (man. transmission), slowly open propane valve while observing tachometer. Continue to open valve until RPM begins to drop, and note maximum enriched idle RPM.
8. If maximum enriched idle RPM is within specifications, proceed to Step 15. The enriched idle RPM for 1983 models is 20 RPM. For 1984 models the enriched idle RPM is 50 RPM for all models except Cab-Chassis. For 1984 Cab-Chassis, enriched idle RPM is 60 RPM.
9. If enriched RPM is not within specification, remove carburetor, then the concealment plug. Refer to appropriate carburetor section for ``Concealment Plug Removal'' for procedure. Reinstall carburetor.
10. Turn mixture screw inward (clockwise) until lightly seated, then back out 3 turns.
11. Start engine, then place transmission in Drive (manual transmission in Neutral).
12. Back out mixture screw 1/8 turn at a time until maximum idle speed is obtained, then set idle speed to maximum enriched idle RPM. Maximum idle speed is idle RPM plus propane enrichment RPM.
13. Turn mixture screw inward (clockwise) until specified curb idle speed is obtained, then recheck maximum enriched RPM. If not within specifications, repeat steps above.
14. Check and, if necessary, adjust fast idle speed.
15. Turn ignition off, remove propane tool, then reconnect crankcase ventilation and vacuum hoses.