Retardation of the ignition timing can be used to control the emission of oxides of nitrogen and hydrocarbons. The 1970-73 (automatic transmission) models have a dual point distributor to provide advanced and retarded distributor characteristics. The 1974 (automatic transmission) models and 1975-76 (49 States and Canada) models use a distributor with dual pick-ups for the same purpose.
The distributor has two sets of breaker points or two electronic ignition pick-ups which operate independently of each other and are positioned with a relative phase angle of 10° (1970-73), 7° (1974) or 6° (1975-76) apart. This makes one set the advanced set and the other the retarded set.
The two sets are connected in parallel to the primary side of the ignition circuit. One set of points or pick-up controls the firing of the spark plugs and hence the ignition timing, depending on whether or not the retarded set is energized.
When both sets are electrically energized, the first set to open has no control over breaking the ignition coil primary circuit because the retarded set is still closed and maintaining a complete circuit to ground. When the retarded set opens, the advanced set is still open and the primary circuit is broken causing the electromagnetic field in the ignition coil to collapse and the ignition spark to be produced.
When the retarded set is removed from the primary ignition circuit through the operation of a distributor relay inserted into the retarded circuit, the advanced set controls the primary circuit.
See Figures 1, 2, 3, 4, 5, 6 and 7
- On 1970-73 models, disconnect the distributor wiring harness from the engine harness. Connect the black wire of the engine harness to the black wire of the distributor harness using a jumper wire. This connects the advanced set of points. On 1974 models, disconnect the engine harness from the coolant temperature switch. This activates the advanced pick-up. On 1975-76 models, disconnect the red engine harness wire from the coolant temperature switch. Ground the engine harness wire to the engine block (use a jumper wire if necessary).
- With the engine at normal operating temperature and idling at the specified idling speed, adjust the engine timing to specification.
- On 1970-73 models, disconnect the jumper wire from the black wire of the distributor harness and connect it to the yellow wire of the distributor harness. This connects the retarded set of points. To connect the retarded pick-ups on the 1974 models, use a jumper wire to short circuit the engine harness wire disconnected from the temperature switch in Step 1. This will short circuit the advance control relay. On the 1975-76 models, remove the ground from the engine harness red wire, leaving the wire disconnected.
With the engine idling, check the ignition timing. It should be retarded from the advanced setting as follows:
1970-73 models: 10°
- To adjust the phase angle of the ignition timing, loosen the adjuster plate set screws on the same side as the retarded point or pick-up set.
- Place the blade of a screwdriver in the notch of the adjuster plate and move the adjuster plate as required to obtain the correct retarded ignition timing specification. The timing is retarded when the adjuster plate is turned counterclockwise. There are graduations on the plate to make the adjustment easier: one graduation is equal to 4° of crankshaft rotation.
- Replace the distributor cap, start the engine and check the ignition timing with the retarded side activated (as in Step 3).
- Repeat Steps 6 and 7 as necessary to properly set the retarded ignition timing.