Some models are equipped with a dual-diaphragm distributor unit. This distributor has a retard diaphragm, as well as a diaphragm for advance.
Retarding the timing helps to reduce exhaust emissions, as well as making up for the lack of engine braking on models equipped with a throttle positioner.
- Connect a timing light to the engine. Check the ignition timing.
Before proceeding with the tests, disconnect any spark control devices, distributor vacuum valves, etc. If these are left connected, inaccurate results may be obtained.
- Remove the retard hose from the distributor and plug it. Increase the engine speed. The timing should advance. If it fails to do so, then the vacuum unit is faulty and must be replaced.
- Check the timing with the engine at normal idle speed. Unplug the retard hose and connect it to the vacuum unit. The timing should instantly be retarded from 4-10°deg;. If this does not occur, the retard diaphragm has a leak and the vacuum unit must be replaced.