Air/Fuel Mixture: Adjustments

ROCHESTER QUADRAJET 4 bbl

NOTE: On a new vehicle, less than 300 miles, reduce rpm settings by 75 rpm.

NOTE: Tampering with the carburetor is a violation of Federal law. Adjustment of the carburetor idle air-fuel mixture can only be performed under the following circumstances:
- If an idle defect exists after normal diagnosis reveals no faulty conditions.
- After a major carburetor overhaul. Upon completion of adjustment, concealment plugs must be replaced.

Concealment Plug Removal:

1. Remove concealment plugs as follows:
a. Remove carburetor from vehicle.
b. Turn the carburetor so the bottom is facing up, and use a hacksaw to make two parallel cuts in the throttle body. Make cuts on each side of the locator points beneath each concealment plug. Cut down to the plugs and do not extend the cut further than 1/8" past the locator points.
c. Position a flat punch near the end of the saw marks in the throttle body. Hold the punch at a 45° angle, and strike with a hammer until the casting breaks away. Now the concealment plug is exposed.
d. Use a small pin punch and drive the concealment plug out of the throttle body.
e. Repeat procedures for the other side.
f. Install carburetor.
2. Set parking brake and place transmission in neutral.
3. Turn off all lights and accessories, and connect a tachometer to the engine.
4. Start engine and allow to warm up by idling at second highest step on fast idle cam. When engine reaches normal temperature, return to idle and shut engine off.
5. Disconnect and plug vacuum hoses from EGR valve, distributor, and cannister purge hose at cannister.

Propane Supply Tool:

6. Disconnect the vacuum supply hose from the choke diaphragm at the carburetor and connect the propane supply hose in its place. Make sure that all other vacuum connectors remain intact.
7. Apply vacuum to the HIAS door diaphragm, which opens the door.
8. Make sure that propane supply remains upright during entire procedure.
9. Remove PCV valve from valve cover and allow to draw underhood air.
10. Start engine and allow to run for at least 2 minutes. Open propane main valve.
11. Slowly open propane metering valve until maximum RPM is reached. Too much propane will cause the RPM to drop, fine tune the metering valve to get maximum RPM.

Idle Speed Screw:

12. With propane still flowing, adjust idle speed screw to obtain specified propane RPM. See SPECIFICATIONS for propane enrichment rpm specifications.
13. Adjust propane metering valve again to obtain maximum RPM. If there has been a change in idle speed RPM, readjust idle speed screw to propane RPM specifications.
14. Turn off propane valve and allow engine RPM to stabilize.
15. Slowly adjust mixture screws 1/16 of a turn at a time, allowing 30 seconds between adjustments for engine speed to stabilize. Adjust for smoothest idle at specified curb idle RPM.
16. Turn on main propane valve and adjust metering valve to obtain maximum engine RPM. If maximum engine speed differs by more than 25 RPM from specified propane RPM, repeat steps 10 through 15.
17. When adjustment is correct, turn off both propane valves, stop engine and remove propane supply.
18. Install PCV valve and all vacuum hoses.
19. Install concealment plugs, then perform "Curb Idle Speed, Adjust" and "Fast Idle Speed, Adjust" procedures.