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.
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° BTDC, the spark plug must fire 5° 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 two means to advance the timing of the spark as the engine speed increases. This is accomplished by centrifugal weights within the distributor, and a vacuum diaphragm mounted on the side of the distributor.
If the ignition is set too far advanced (BTDC), the ignition and expansion of the fuel in the cylinder will occur too soon and tend 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. This will result in poor engine performance and lack of power.
Timing marks consist of a notch on the rim of the crankshaft pulley and a scale of degrees attached to the front of the engine. The notch corresponds to the position of the piston in the number 1 cylinder. A stroboscopic (dynamic) timing light is used, which is hooked into the circuit of the No. 1 cylinder spark plug. Every time the spark plug fires, the timing light flashes. By aiming the timing light at the timing marks, the exact position of the piston within the cylinder can be read, since the stroboscopic flash makes the mark on the pulley appear to be standing still. Proper timing is indicated when the notch is aligned with the correct number on the scale.
There are three basic types of timing light available. The first is a simple neon bulb with two wire connections (one for the spark plug and one for the plug wire, connecting the light in series). This type of light is quite dim, and must be held closely to the marks to be seen, but it is quite inexpensive. The second type of light operates from the car's battery. Two alligator clips connect to the battery terminals, while a third wire connects to the spark plug with an adapter. This type of light is more expensive, but the xenon bulb provides a nice bright flash which can even be seen in sunlight. The third type replaces the battery source with 110 volt house current. Some timing lights have other functions built into them, such as dwell meters, tachometers, or remote starting switches. These are convenient, in that they reduce the tangle of wires under the hood, but may duplicate the functions of tools you already have.
Because your X-Body car has electronic ignition, you should use a timing light with an inductive pickup. This pickup simply clamps around the Number 1 spark plug wire, eliminating the adapter. It is not susceptible to crossfiring or false triggering, which may occur with a conventional light due to the greater voltages produced by HEI.INSPECTION & ADJUSTMENT
- Refer to the directions on the tune-up label inside the engine compartment. Follow all the instructions on the label.
- Locate the timing marks on the crankshaft pulley and the front of the engine.
- Clean off the timing marks so that you can see them. Use chalk or white paint to color the mark on the crankshaft pulley and the mark on the scale which will indicate the correct timing when aligned with the notch on the crankshaft pulley.
- Attach a tachometer to the engine. See the preceding information, "HEI System Tachometer Hookup".
- Attach a timing light to the engine, according to the manufacturer's instructions. If the timing light has three wires, one, usually green or blue, is attached to the No. 1 spark plug with an adapter, unless an inductive pickup is used, which simply clamps around the wire. The other wires are connected to the battery. The red wire goes to the positive side of the battery and the black wire is connected to the negative side of the battery. Do not pierce the No. 1 spark plug wire, or attempt to insert a wire between the boot and the wire. This will break the insulation and result in an ignition miss.
Number one spark plug is at the front of the four cylinder engine (right side of the car) and at the right front of the V6 engine (the left rear spark plug if you are facing the car). Firing order diagrams are in this information.
- Disconnect and plug the vacuum hose at the distributor vacuum advance, if so directed by the tune-up label. This will be required in most cases to prevent vacuum advance to the distributor. The hose must be plugged to prevent a vacuum leak into the carburetor. A golf tee makes a good plug. Be careful not to split the hose.
1981 and later models with Electronic Spark Timing have no vacuum advance, therefore you may skip this step. On Models with (EST) distributor, disconnect the 4 terminal plug at the distributor.
- Check to make sure that all of the wires clear the fan and then start the engine. Allow the engine to reach normal operating temperature.
- Make sure the engine is idling at the correct speed.
- Aim the timing light at the timing marks. If the marks which you put on the pulley and the engine are aligned when the light flashes, the timing is correct. Turn off the engine and disconnect the timing light and tachometer. If the marks are not in alignment, the timing will have to be adjusted.
- Turn off the engine.
- Loosen the distributor lockbolt so that the distributor can just be turned with a little effort. The V6 engine has a conventional lockbolt and clamp. The four cylinder engine has two lockbolts. One, at the outer edge, is the one you want. This loosens the distributor hold-down clamp but does not permit removal of the distributor itself. Loosen the outer lockbolt and slide the clamp away from the distributor slightly. This will allow the distributor to rotate.
- Start the engine. Keep the wires of the timing light clear of the fan and pulleys. While observing the timing marks with the light, turn the distributor slightly until the timing marks are aligned.
- Turn off the engine and tighten the distributor lockbolt. Start the engine and recheck the timing. Sometimes the distributor moves slightly during the tightening process. If the ignition timing is within 1° of the correct setting, that's close enough; a tolerance of 2° is permitted by the manufacturer.
- Shut off the engine and disconnect the timing light and tachometer. Reconnect the distributor vacuum advance hose, if removed.