System Performance Check - Carbureted Models

Fig. 018 - Chart - System Performance Check:

Fig. 020 - Procedure for System Performance Check:

SYSTEM PERFORMANCE CHECK

1. Checks for ability of carburetor main metering system to change the Air/Fuel mixture. Disconnecting M/C solenoid makes carburetor operate full-rich and reconnecting it with the dwell lead grounded makes it operate full-lean. Normal response - RPM drops as M/C solenoid is reconnected. Usually 400-1000 RPM, but should be at least 300 RPM.
1a. Some M/C solenoids are polarity sensitive and will not pull solenoid plunger down if polarity is reversed.

If plugging the PCV, Purge, or Bowl Vent vacuum hose causes RPM to drop over 300 RPM, that hose leads to the source of the problem. If RPM increases as M/C solenoid is connected, it indicates the system is running extremely rich. This can sometimes be caused by incorrect valve timing.

2. Checks for proper control of idle circuit.
2a. This is a full-rich command to the carburetor and can be caused by:
1. Lean engine condition.
2. Grounded oxygen sensor wire or bad sensor.
3. Open in wire from ECM term. "14" to ground.
4. Open wire to ECM term. "22".
5. Open in coolant sensor circuit.
2b. There is an open loop condition. It can be caused by:
1. An open oxygen sensor circuit or bad sensor.
2. An open in coolant sensor circuit.
3. An open in wire from ECM term. "14" to ground.
2c. This is a full lean command. It can be caused by:
1. Rich engine condition caused by:
a. M/C Solenoid wire connections reversed.
b. Leaking Bowl Vent valve, excessive fuel in vapor canister, fuel in crankcase, faulty carburetor calibration or carburetor.
2d. Normal reading - operates in closed loop - dwell is between 10°-50°, but varying. Running for 1 minute at fast idle is to make sure that oxygen sensor is warm.
3. Checks for proper control of main metering system. RPM must be at least 3000 to get into the main metering system operation. Removing and plugging the hose may set a Code 34 so memory will have to be cleared.
3a. A missing "O" ring between the switching valve solenoid and the valve, or a faulty valve, may cause air to leak to the exhaust ports at higher RPM only.