Normally Open Air Bypass Valves With Vacuum Vents

1. Disconnect the air pump supply line at the outlet.

2. Disconnect all vacuum lines from the vacuum nipple and the vacuum vent (Fig. 2).

3. With the engine at 1500 rpm, air pump supply air should be heard and felt at the outlet (Fig. 2)

4. Using a length of vacuum hose with no restrictors or devices, connect the vacuum nipple to one of the manifold vacuum fittings on the intake manifold. With the vacuum vent open to atmosphere and the engine at 1500 rpm, virtually no air should be felt at the valve outlet and virtually all air should be bypassed through the silencer ports.

5. Using the same direct vacuum line to an intake manifold vacuum source, cap the vacuum vent. Accelerate the engine to 2000 rpm, and suddenly release the throttle. A momentary interruption of air pump supply air should be felt at the valve outlet (Fig. 2)

6. Reconnect all vacuum and thermactor lines. If any of the above tests are not satisfactorily completed, check the air pump. If the air pump is operating satisfactorily, replace the air bypass valve.