Idle Mixture Adjustment Procedure
(PERFORM ON ALL MANUAL TRANSMISSION VEHICLES)1. Remove air cleaner housing:
a. Remove bolt and nut (Figure 2, 1).
b. Disconnect the following hoses:
- AS (Air Suction) hose (Figure 2, 5)
- HAI (Hot Air Intake) hose ((Figure 2, 3)
- PCV (Positive Crankcase Ventilation) hose (Figure 2, 4)
- HIC (Hot Idle Compensator) hose (Figure 2, 2)
c. Remove air cleaner housing.
2. Disconnect accelerator cable from carburetor.
3. Disconnect carburetor wiring connectors and attach tags to identify where connectors should be reconnected.
4. Disconnect the following hoses from carburetor:
a. Emission control hoses.
b. Fuel inlet hose.
C. Outer vent hose.
NOTICE:
Before disconnecting these hoses attach tags to identify where hoses should be reconnected (Figure 18). The vacuum hose routing label and service manual diagrams may also be used to identify proper hose connections.
5. Remove carburetor as follows:
NOTICE:
DO NOT SMOKE OR WORK NEAR AN OPEN FLAME WHEN SERVICING CARBURETOR!
a. Remove carburetor mounting nuts.
b. Remove cold mixture heater wire clamp and lift out EGR vacuum modulator bracket.
c. Lift out carburetor.
d. Remove old carburetor gasket.
e. Cover inlet hole of intake manifold with a clean shop rag.
6. Remove mixture adjusting screw plug (MAS plug) from carburetor using the following procedure:
a) Plug each carburetor vacuum port to prevent entry of steel particles when drilling.
b) Prick punch center of MAS plug.
c) Drill a 6.5 mm (0.256 in.) hole in center of plug (Figure 19) being careful not to drill down onto mixture adjustment screw.
NOTICE:
THERE IS ONLY 1 mm (0.04 in.) CLEARANCE BETWEEN MAS PLUG AND THE MIXTURE ADJUSTMENT SCREW. DRILL CAREFULLY AND SLOWLY TO AVOID DRILLING INTO SCREW.
The drill may have forced MAS plug out completely at this time. If MAS plug has been forced out, go on to step 7. If MAS plug has not been forced out, continue to step 6d.
d. Through hole drilled in MAS plug, fully screw in mixture adjusting screw.
NOTICE:
BE CAREFUL NOT TO DAMAGE SCREW TIP BY TIGHTENING SCREW DOWN TOO TIGHTLY.
e. Use a 7.5 mm (0.295 in.) drill to remove remainder of MAS plug (Figure 20).
7. Using compressed air, blow any steel particles away from mixture adjusting screw.
8. Remove mixture adjusting screw and inspect for damage (Figure 21 shows screw in proper condition). If drill has gnawed into screw, or if tapered portion of screw is damaged, replace screw.
9. Fully screw in mixture adjusting screw and then unscrew it 3 and 1/4 revolutions (Figure 22).
NOTICE:
BE CAREFUL NOT TO DAMAGE SCREW TIP BY TIGHTENING SCREW DOWN TOO TIGHTLY.
10. Reinstall carburetor using new gaskets from kit as shown in Figure 23. Tighten 4 bolts to 15 N-m (11 lb.ft.).
11. Reconnect carburetor vacuum hoses to proper locations referring to tags attached in step 4 and to vacuum hose routing label located under hood (or refer to 1988 Nova Service Manual).
12. Reinstall air cleaner.
a. Connect the following hoses:
- HIC (Hot Idle Compensator) hose (Figure 2, 2)
- PCV (Positive Crankcase Ventilation) hose (Figure 2, 4)
- HAI (Hot Air Intake) hose (Figure 2, 3)
- AS (Air Suction) hose (Figure 2, 5)
b. Install air cleaner housing with bolt and wing nut.
13. Adjust idle speed and mixture as follows:
a. These initial conditions must be met:
- Air cleaner installed
- Normal operating coolant temperature
- Choke fully open
- All accessories switched off
- All vacuum lines connected
- Ignition timing set correctly
- Transmission in neutral
- Fuel level should be even with correct level mark in carburetor sight glass
- Front wheels in straight ahead position for power steering equipped vehicles
b. Start engine.
c. Set to maximum idle speed by turning idle mixture adjusting screw (Figure 24-A). This screw can be reached by inserting a screwdriver between EGR valve and EGR vacuum modulator bracket (Figure 24-B).
d. Set idle speed by turning idle speed adjusting screw (Figure 25). Idle speed should be 700 RPM.
e. Continue to adjust idle mixture screw and idle speed screw until maximum speed will not rise any further no matter how much the idle mixture adjusting screw is adjusted.
f. Set to 650 RPM by screwing in idle mixture adjusting screw.
Notice:
This is the lean drop method for setting idle speed and mixture.
14. Plug idle mixture adjusting screw as follows:
a. Remove air cleaner and EGR vacuum modulator bracket.
b. With tapered end inward, tap in new MAS plug (from HAC kit) until it is flush with carburetor surface (Figure 26).
c. Reinstall EGR vacuum modulator bracket and air cleaner.
15. Install Campaign Identification Label. For vehicles which were previously corrected under the original campaign 88C30 repair, install a second Campaign Identification Label identifying 88C32.