The dual diaphragm distributor has two diaphragms which operate independently. The outer (primary) diaphragm makes use of carburetor vacuum to advance the ignition timing. The inner (secondary) diaphragm uses intake manifold vacuum to provide additional retardation of ignition timing during closed throttle deceleration and idle, resulting in the reduction of hydrocarbon emissions.
- To check vacuum advance, disconnect the vacuum lines from both the advance (outer) and retard (inner) diaphragms. Plug the line removed from the retard diaphragm.
- Connect a tachometer and timing light to the engine. Increase the idle speed by setting the screw on the first step of the fast idle cam. Note the ignition timing setting, using a timing light.
- Connect the carburetor vacuum line to the advance diaphragm. If the timing advances immediately, the advance unit is functioning properly. Adjust the idle speed to 550-600 rpm.
- Check the vacuum retardation as follows: using a timing light, note the ignition timing. Remove the plug from the manifold vacuum line and connect the line to the inner diaphragm. Timing should retard immediately.
- If vacuum retardation is not to specifications, replace the dual diaphragm advance unit. If the advance (vacuum) does not function properly, calibrate the unit on a distributor test stand. If the advance part of the unit cannot be calibrated, or if either diaphragm is leaking, replace the dual diaphragm vacuum advance unit.