- 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 idle speed by setting the screw on the first step of the fast idle cam. Note 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.