1974 Models
- Disconnect vacuum hose between mixture control valve and vacuum switching valve (at vacuum switching valve). Start engine, place hand over mixture control valve inlet port, and check for vacuum.
- If vacuum is felt, mixture control valve is defective. Apply vacuum to disconnected vacuum hose, air should momentarily flow through mixture control valve. See Figure.
- With vacuum hose still disconnected, connect a vacuum gauge to vacuum switching valve and position gauge so it may be seen while driving vehicle. Road test vehicle.
- Increase vehicle speed to at least 45 MPH. At 39-43 MPH, vacuum should be indicated on vacuum gauge. Slowly allow vehicle to decelerate. At 26-36 MPH, vacuum gauge reading should drop to zero (0).