Distributor Modulator: Testing

System should be checked when loss of engine performance and excessive fuel consumption are reported. Road test symptoms will be those of retarded ignition timing. To check the system:

1. Start engine, run at 1500 RPM and check for vacuum at carburetor. Note vacuum reading and reconnect vacuum hose.
2. By-pass distributor modulator hoses at rear of engine and connect vacuum gauge to distributor primary, observe vacuum reading. If vacuum is not apparent or is considerably less than in step  1, check for pinched and/or leaking hoses (or lines) on engine.
3. If vacuum in step  2 is approximately the same as in step  1, remove by-pass and connect vacuum hoses from firewall to rear of engine. Run engine until warm and at normal idle.
   NOTE: Make sure thermal switch is above 65°F.

   Raise rear wheels, slowly accelerate to 34 MPH. If vacuum occurs between 23-30 MPH, system is functioning properly.

4. If vacuum occurs before 20 MPH or at start of acceleration, check electrical connections inside vehicle leading to modulator box. Disconnect thermal switch and recheck vacuum. If vacuum is present, replace modulator assembly. If no vacuum, replace thermal switch.
5. If there was no vacuum in step  3, check electrical and hose connections inside vehicle leading to modulator box. If connections are unsatisfactory, repair as required. If connections are satisfactory, replace distributor modulator assembly and recheck vacuum. If still no vacuum, replace sensor.