System Performance Check (CARBURETED): Notes

This verifies the computerized engine control system is functioning correctly and should always be made after any repair of computerized engine control system. Ensure parking brake is engaged and DRIVE wheels are blocked. Parking brake on FWD models does NOT hold drive wheels.

1. Start engine and warm to operating temperature. Run engine at 2000 RPM for one minute to ensure O2 sensor is warm. Connect tachometer to engine. Ground diagnostic test terminal "B". See Figure . Disconnect mixture control solenoid and ground Green mixture control dwell lead. Increase engine speed to 3000 RPM and hold throttle held steady, while reconnecting mixture control solenoid. Note RPM change. This checks carburetor's ability to change air/fuel mixture. Disconnecting M/C solenoid makes carburetor run full rich. Reconnecting it with dwell lead grounded makes carburetor run full lean. RPM normally drops 300-1000 RPM as solenoid is reconnected. If RPM drops as described, go to step 3). Remove ground from dwell lead before returning to idle.
2. If RPM dropped less than 300 RPM or RPM increased in step 1), check evaporative canister and related valves for fuel saturation. If plugging PCV, purge or bowl vent hose causes RPM to drop more than 300 RPM, the hose that caused the RPM to change will lead to source of problem. If RPM increases as M/C solenoid is connected, system is running extremely rich. If no faults are found during system checks, service carburetor.
3. Connect dwell meter (on 6-cylinder scale) to Green mixture control solenoid dwell lead. Set carburetor on highest step of fast idle cam. Run for one minute or until dwell begins to vary, whichever happens first. Allow engine to idle and note dwell. This checks for proper control of idle circuit. Normal dwell will vary between 10 and 50 degrees, indicating "closed loop" operation. If dwell does NOT vary between 10 and 50 degrees, see appropriate heading:
   • Dwell Fixed Less Than 10 Degrees
   • Dwell Fixed Between 10 & 50 Degrees
   • Dwell Fixed Greater Than 50 Degrees
4. If dwell is varying between 10 and 50 degrees, observe dwell at 3000 RPM to check for proper control of main metering system. RPM must be at least 3000 RPM to enter main metering system operation. Normal response is varying between 10 and 50 degrees. If dwell is less than 10 degrees, check air injection system for valve supplying air to exhaust manifold at 3000 RPM. A missing "O" ring between the switching valve solenoid and valve, or a defective valve, may cause air to leak into the exhaust ports at higher RPM only. If air is not being supplied to exhaust or dwell at 3000 RPM is greater than 50 degrees, service internal carburetor problems.