Propane-Assisted Idle Mixture Procedure

Tampering with the carburetor is a violation of Federal law. Adjustment of the carburetor idle air-fuel mixture can only be done under certain circumstances as explained below.
This procedure should only be used if idle defect still exists after normal diagnosis has revealed no other faulty condition, such as, incorrect idle speed, incorrect basic timing, faulty hose or wire connections, etc. It is also important to make sure the combustion computer systems are operating properly. Adjustment of the air-fuel mixture should also be performed after a major carburetor overhaul.

1. Remove concealment plugs (refer to Service Manual).

2. Turn off all lights and accessories.

3. Place transmission in neutral or park.

4. Set the parking brake.

5. Start and run the engine until operating temperature is reached.

6. Ground the carburetor switch with a jumper wire.

7. Disconnect and plug the vacuum hoses at
(a) Computer, All Except 3.7L Federal.
(b) EGR valve
(c) Canister (3/16" diameter one)

8. Remove PCV valve from the valve cover and allow it to draw outside air.

9. Connect a tachometer to engine.

10. Tee in propane supply hose in the vacuum line to the choke vacuum kick diaphragm.

11. Open the propane main valve. Slowly open the propane metering valve until maximum engine speed is reached.

NOTE: When too much propane is added, engine speed will decrease. Fine tune the metering valve for the highest engine rpm. Also, if idle mixture is extremely rich, engine rpm will decrease with any amount of propane. In this case turn the idle mixture screw in about 1/2 turn and repeat step

12. With the propane still flowing, adjust the idle speed screw on the solenoid until the tachometer indicates the specified propane rpm.

13. Fine tune the metering valve for the highest engine rpm. If there has been a change in the maximum rpm, re-adjust the idle speed screw to the specified propane rpm.

14. Turn off the main propane valve and allow the engine speed to stabilize.

15. Slowly adjust the mixture screw(s) until the tachometer indicates the specified idle set rpm. Pause between adjustment to allow the engine speed to stabilize.

16. Turn on the propane valve.
17. Fine tune the metering valve to get the highest engine rpm. If maximum speed is more than 25 rpm different than the specified propane rpm, repeat steps 12 through 18. If okay, proceed to step 20.

18. Turn off both valves on the propane bottle.

19. Reinstall PCV into valve cover.

20. Reconnect computer, EGR, and canister vacuum hoses.

21. Reconnect computer hose, all except 3.7L Federal.

22. Move jumper wire from carb switch.

NOTE: The idle speed with the engine in normal operating condition (everything connected) may vary from set speeds. DO NOT READJUST.

23. Remove tachometer and turn engine off.

24. Remove propane supply hose.

25. Reinstall the concealment plugs.