DRIVEABILITY AT LIGHT THROTTLE HESITATION. Chevrolet Nova 1986

ENGINE DRIVEABILITY AT LIGHT THROTTLE APPLICATIONS

VEHICLES AFFECTED: 1985-87 "S" CAR

Some 1985-87 vehicles prior to VIN HZ063093, may continue to exhibit hesitation and sag on light acceleration after performing the corrective action steps described in the previous Bulletin #87-616-100.

On vehicles where customers continue to experience the symptoms described after the application of the Bulletin, carry out the operations listed in the following table.

STEP OPERATION A/T M/T ---- --------- --- --- 1 Float Level Adjustment Yes Yes 2 Remove Carburetor Yes Yes 3 Reset Mixture Adjusting Screw Yes Yes 4 Remove Air Horn Assembly No Yes 5 New Power Piston Spring No Yes 6 New Primary Main Jet Yes Yes 7 New Auxiliary Accelerator Pump (AAP) No Yes 8 Reassemble Air Horn No Yes 9 New Accelerator Pump Arm and Rod Yes Yes (1986-7 Only) 10 New Secondary Throttle Valve Diaphragm Yes Yes Spring 11 Adjust Secondary Throttle Valve Kick-Up Yes Yes Clearance 12 Adjust Secondary Choke Breaker Angle No Yes 13 Reinstall Carburetor Yes Yes 14 New HAC/EBCV Jet (Federal Only) Yes Yes 15 Additional Vacuum Transmitting Valve Yes Yes (VTV) 16 Adjust Accelerator Pump Stroke Yes Yes 17 Check and Adjust Engine To Spec. Yes Yes 18 Install idle mixture screw plug Yes Yes 19 Vacuum Hose Information Label Yes Yes

Dependent upon the model year and emission system installed on the complaint vehicle, a kit is available by ordering from the following list of "Jet Kit" part numbers, at the same time P/N 94840180, plug carburetor should be ordered.

Model Year Emission System Transmission P/N ---------- --------------- ------------ --- 1985 (F) Federal Automatic 94845213

1986-7 (G/H) Federal Automatic 94845214

1986-7 (G/H) Calif./Canada Automatic 94845215

1985 (F) Federal Manual 94845216

1986-7 (G/H) Federal Manual 94845217

Service Procedure:

1. Float level adjustment (A/T and M/T). Float level should be adjusted such that the fuel level is above the mid-point of the range of the sight glass. If necessary adjust float level following service manual procedures.

2. Remove carburetor following service manual procedures (A/T and M/T).

3. Reset mixture adjusting screw (A/T and M/T).

o Remove the mixture adjusting screws (MAS) plug following service manual procedures.

o Unscrew mixture adjusting screw three (3) complete turns.

4. Remove air horn assembly following service manual procedure (M/T only).

5. New Power Piston Spring (M/T only) . Remove power piston spring and replace with new spring contained in jet kit following service manual procedures.

6. New Primary Main Jet (A/T and M/T).

Remove the primary main jet (and copper gasket) and install new jet and gasket from kit. Follow service manual procedures.

7. New Auxiliary Accelerator Pump (AAP) (M/T only).

Remove the AAP housing and spring and install new housing and spring from kit. Follow service manual procedures.

8. Re-assemble air horn to carburetor (M/T only). See service manual procedure.

9. 1986-7 Only - New Accelerator Pump Arm and Rod (A/T and M/T).

Remove accelerator pump arm and rod and install new parts from kit following service manual instructions.

10. New Secondary Throttle Valve Diaphragm Spring (A/T and M/T).

Remove the secondary throttle valve diaphragm following service manual procedures.

Disassemble the diaphragm assembly by removing the four (4) cover retaining screws, lift off the cover casting and remove the spring from the top of the diaphragm. Install new diaphragm spring from kit and reassemble diaphragm assembly and reinstall on carburetor.

11. Adjust Secondary Throttle Valve Kick - Up Clearance (A/T and M/T). Following service manual procedures adjust the secondary throttle valve kick-up clearance to a new setting of .55 - .65mm (.022 - .025 in.) - (was .015mm/.006 in.). After adjustment - position the primary throttle valve in the idle position and check that the secondary throttle valve is fully closed.

12. Adjust Secondary Choke Breaker Angle (M/T only). Following service manual procedures, after applying vacuum to choke breaker diaphragms 1st and 2nd, adjust choke valve angle to 58 degrees from horizontal (was 55 degrees).

13. Reinstall carburetor following service manual procedures (A/T and M/T).

14. New HAC/EBCV Jet - Federal only (A/T and M/T).

Disconnect hoses from jet (6 way hose connector) between the high altitude compensator (HAC) valve, the electronic air bleed control valve (EBCV) and the carburetor and install new jet from kit.

15. Additional vacuum transmitting valve (A/T and M/T).

Insert the vacuum transmitting valve (VTV) from kit in the vacuum hose between the EGR vacuum modulator "Q" port and the EGR valve. The VTV housing has two colors - the black side of the VTV must be connected to the hose from the vacuum modulator.

16. Adjust Accelerator Pump Stroke (A/T and M/T).

With engine not running, remove air cleaner assembly. Unscrew the idle speed adjusting and throttle positioner (TP) screws until the throttle valve is fully closed. With the choke valve fully open, measure the length of the accelerator pump stroke. If necessary adjust stroke to 4.0mm (.157in) by bending the connecting link.

17. Check and Adjust Engine To Specifications (A/T and M/T).

Following service manual procedures check and adjust the following to specifications shown on the underhood emission control information label and in the service manual: Engine Timing, Idle Mixture, Idle Speed, Throttle Positioner (TP) , Throttle Position Switch (TPS) , Fast Idle Speed, and A/C Idle-Up Speed when applicable.

18. Plug idle mixture screw (A/T and M/T) following service manual procedures using plug, P/N 94840180.

19. Vacuum Hose Information Label (A/T and M/T).

Remove existing underhood vacuum hose information label and install new label from kit.

NOTICE: THE KITS FOR CALIFORNIA EMISSIONS EQUIPPED CARS INCLUDE DIFFERENT VACUUM HOSE INFORMATION LABEL0194S FOR CARS WITH OR WITHOUT POWER STEERING. WHEN APPLYING THE HOSE INFORMATION LABEL, CARE SHOULD BE TAKEN TO ENSURE THAT THE LABEL APPROPRIATE TO THE VEHICLE IS USED.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.