See Figures 1, 2, 3, 4 and 5
Ignition timing is the measurement, in degrees of crankshaft rotation, of the point at which the spark plugs fire in each of the cylinders. It is measured in degrees before or after Top Dead Center (TDC) of the compression stroke. Ignition timing is adjusted by turning the distributor body in the engine.
Ideally, the air/fuel mixture in the cylinder is ignited by the spark plug just as the piston passes TDC of the compression stroke. If this happens, the piston will begin the power stroke just as the compressed and ignited air/fuel mixture starts to expand. The expansion of the air/fuel mixture then forces the piston down on the power stroke and turns the crankshaft.
Because it takes a fraction of a second for the spark plug to ignite the mixture in the cylinder, the spark plug must fire a little before the piston reaches TDC. Otherwise, the mixture will not be completely ignited as the piston passes TDC and the full power of the explosion will not be used by the engine.
The timing measurement is given in degrees of crankshaft rotation before the piston reaches TDC (BTDC). If the setting for the ignition timing is 5 degrees BTDC, each spark plug must fire 5 degrees before each piston reaches TDC. This only holds true, however, when the engine is at idle speed.
As the engine speed increases, the pistons go faster. The spark plugs have to ignite the fuel even sooner if it is to be completely ignited when the piston reaches TDC. To do this, the distributor has a means to advance the timing of the spark as the engine speed increases. This is often accomplished by centrifugal weights within the distributor and a vacuum diaphragm assembly mounted on the side of the distributor.
DuraSpark III and EEC-IV Thick Film Integrated (TFI) ignition systems do not utilize centrifugal or vacuum advance, and thus, do not utilize centrifugal weights or vacuum diaphragm assemblies.
It is necessary to disconnect the vacuum line(s) from the diaphragm assembly (if so equipped) when the ignition timing is being set.
If the ignition is set too far advanced (BTDC), the ignition and expansion of the fuel in the cylinder will occur too soon and try to force the piston down while it is still traveling up. This causes engine ping. If the ignition spark is set too far retarded after TDC (ATDC), the piston will have already passed TDC and started on its way down when the fuel is ignited. This will cause the piston to be forced down for only a portion of its travel, resulting in poor engine performance and a lack of power.
The timing is best checked with a timing light. This device is usually connected in series with the No. 1 spark plug. The current that fires the spark plug also causes the timing light to flash.
The crankshaft pulley on 6-200 and 6-232 engines contains a notch. A scale of degrees of crankshaft rotation is attached to the engine block in such a position that the notch will pass close by the scale. On 4-140, 6-170 and V8 engines, the scale is located on the crankshaft pulley and a pointer is attached to the engine block so that the scale will pass close by. When the engine is running, the timing light is aimed at the mark on the crankshaft pulley and the scale.
See Figures 6 and 7
All engines are equipped with conventional ignition timing marks and pointers. In addition, some early model engines are equipped with a monolithic timing system. The monolithic system uses a timing receptacle, which accepts an electronic probe that is connected to digital readout equipment. When equipped, this receptacle is located at the front of most engines (except the 4-140, which has a boss for monolithic timing at the left rear of the cylinder block). Timing can also be adjusted in the conventional way. Many 1980 and later models are equipped with EEC engine controls, in which all ignition timing is controlled by the EEC module. Initial ignition timing is not adjustable and no attempt at adjustment should be made on EEC equipped vehicles. Since requirements vary from model to model, always refer to the Vehicle Emission Control Information (VECI) label for details concerning your particular vehicle.
- Locate the timing marks and pointer on the lower engine pulley and engine's front cover.
- Clean the marks and apply chalk or brightly colored paint to the pointer.
- On 1981 and later models, if the ignition module has (-12A244-) as a basic part number, disconnect the two wire connector (yellow and black wires). On engines equipped with the EEC-IV system, disconnect the single white (black on some models) wire connector near the distributor.
- Attach a timing light and tachometer according to manufacturer's specifications.
The coil connector used with DuraSpark is provided with a cavity for connection of a tachometer, so that the connector does not have to be removed to check engine rpm. Install the tachometer lead with an alligator clip on its end into the cavity marked TACH TEST and connect the other lead to a good ground. If the coil connector must be removed, pull it out horizontally until it is disengaged from the coil terminal.
- Disconnect and plug all vacuum lines leading to the distributor.
- Start the engine, allow it to warm to normal operating temperature, then set the idle to the specifications given on the underhood sticker (for timing).
- On 1981 and later models equipped with the module mentioned in Step 3, jumper the pins in the module connector for the yellow and black wires.
- Aim the timing light at the timing mark and pointer on the front of the engine. If the marks align when the timing light flashes, remove the timing light, set the idle to its proper specification, and connect the vacuum lines at the distributor. If the marks do not align when the light flashes, turn the engine off and loosen the distributor hold-down clamp slightly.
- Start the engine again, and observe the alignment of the timing marks. To advance the timing, turn the distributor opposite of its rotational direction, meaning counterclockwise on 4-140, 6-170 and 6-200 engines, and clockwise on 6-232 and V8 engines. To retard the timing, turn the distributor in its rotational direction, meaning clockwise on 4-140, 6-170 and 6-200 engines, and counterclockwise on 6-232 and V8 engines. When altering the timing, it is wise to tap the distributor lightly with a wooden hammer handle to move it in the desired direction. Grasping the distributor with your uninsulated hand may result in a painful electric shock. When the timing marks are aligned, turn the engine OFF and tighten the distributor hold-down clamp. Remove the test equipment, reconnect the vacuum hoses and white (or black) single wire connector (EEC-IV).
On 1981 and later models equipped with the module mentioned in Step 3, remove the jumper connected in Step 7 and reconnect the two-wire connector. Test the module operation as follows:
- Disconnect and plug the vacuum source hose to the ignition timing vacuum switch.
Using an external vacuum source, apply vacuum greater than 12 in. Hg (40.52 kPa) to the switch, and compare the ignition timing with the requirements below.
4-cylinder: per specifications less 32-40 degrees