Ford- Fairmount/Zephyr 1978-1983 Repair Guide

Dual Diaphragm Distributors



Most Fairmont and Zephyr models are equipped with dual diaphragm distributors. The dual diaphragm is a two-chambered housing which is mounted on the side of the distributor. The outer side is a conventional vacuum advance mechanism, connected to the carburetor by a vacuum hose. The purpose of the vacuum advance unit is to advance ignition timing in response to the conditions under which the engine is operating. Vacuum advance units have been in use for many years, and their chief advantage is economical engine operation.

The inner or second side of the diaphragm helps control exhaust emissions at idle and during deceleration. This inner side of the dual diaphragm is connected by a vacuum hose to the intake manifold. When the engine is idling or decelerating, intake manifold vacuum is high and carburetor vacuum is low. Under these conditions, intake manifold vacuum, applied to the inner side of the dual diaphragm, retards ignition timing to promote more complete combustion of the air/fuel mixture in the engine combustion chambers.


  1. Connect a timing light and start the engine.
  3. Disconnect the vacuum hose from the outer distributor diaphragm. Plug the line with a golf tee or pencil.
  5. Shine the timing light on the lower engine pulley and observe the position of the ignition timing marks.
  7. Disconnect the vacuum hose from the rear vacuum diaphragm on the distributor and hold your finger over the line.
  9. Again check the ignition timing. It should be advanced beyond the reading obtained in Step 3. If it is not, the rear diaphragm on the distributor is defective or the vacuum hose from the intake manifold is broken.


Vacuum Advance/Retard Diaphragm
  1. Tag the two vacuum hoses that attach to the diaphragm and disconnect them.
  3. Remove the distributor cap and remove the C-clip that attaches the diaphragm arm to the breaker point mounting plate.
  5. Remove the two screws that attach the diaphragm to the distributor housing and remove the diaphragm from the distributor.
  7. Position the new diaphragm on the distributor and connect the hole in the diaphragm arm to the stud on the breaker point mounting plate.
  9. Install the C-clip that retains the arm on the plate.
  11. Install the distributor cap.
  13. Install the two diaphragm attaching screws.
  15. Connect the vacuum hoses to the diaphragm.