See Figures 1 and 2
Ignition timing is the measurement (in degrees) of crankshaft position at the instant the spark plug fires. Ignition timing is adjusted by loosening the distributor locking device and turning the distributor in the engine.
It takes a fraction of a second for the plug to completely ignite the mixture in the cylinder. Because of this, the spark plug must fire before the piston reaches TDC (top dead center, the highest point in its travel), if the mixture is to be completely ignited as the piston passes TDC. This measurement is given in degrees (of crankshaft rotation) before the piston reaches top dead center (BTDC). If the ignition timing setting for your engine is 7 degrees BTDC, this means that the spark plug must fire at a time when the piston for that cylinder is 7 degrees before top dead center of its compression stroke. However, this only holds true while your engine is at idle speed.
As you accelerate from idle, the speed of your engine (rpm) increases. The increase in rpm means that the pistons are now traveling up and down much faster. Because of this, the spark plugs will have to fire even sooner if the mixture is to be completely ignited as the piston passes TDC. To accomplish this, the distributor incorporates means to advance the timing of the spark as the engine speed increases.
The distributor in your carbureted vehicle has two means of advancing the ignition timing. One is called centrifugal or governor advance and is actuated by weights in the distributor. The other is called vacuum advance and is controlled by the large circular housing on the side of the distributor.
In addition, some distributors have a vacuum-retard mechanism which is contained in the same housing on the side of the distributor as the vacuum advance. The function of this mechanism is to retard the timing of the ignition spark under certain engine conditions. The causes more complete burning of the air/fuel mixture in the cylinder and consequently lowers exhaust emissions. Because these mechanisms change ignition timing, it is necessary to disconnect and plug the vacuum lines from the distributor when setting the base ignition timing.
The fuel injected vehicles have neither a centrifugal advance nor a vacuum unit. All the timing changes are controlled electronically by the ECM. This solid state "brain'' receives data from many sensors (including the distributor), and commands changes in spark timing (and other functions) based on immediate driving conditions. This instant response allows the engine to be kept at peak performance and economy throughout the driving cycle. Basic timing can still be checked and adjusted on these motors.
If the ignition timing is set too far advanced (BTDC), the ignition and expansion of the air/fuel mixture in the cylinder will try to force the piston down while it is still traveling upward. This causes engine ping, a sound which resembles marbles being dropped into an empty tin can. If the ignition timing is too far retarded (after, or ATDC), the piston will have already started down on the power stroke when the air/fuel mixture ignites and expands. This will cause the piston to be forced down only a portion of its travel. This results in poor engine performance and lack of power.
Ignition timing adjustment is checked with a timing light. This instrument is connected to the number one (No. 1) spark plug of the engine. The timing light flashes every time an electrical current is sent from the distributor through the No. 1 spark plug wire to the spark plug. The crankshaft pulley and the front cover of the engine are marked with a timing pointer and a timing scale.
When the timing pointer is aligned with the 0 mark on the timing scale, the piston in the No. 1 cylinder is at TDC of it compression stroke. With the engine running, and the timing light aimed at the timing pointer and timing scale, the stroboscopic (periodic) flashes from the timing light will allow you to check the ignition timing setting of the engine. The timing light flashes every time the spark plug in the No. 1 cylinder of the engine fires. Since the flash from the timing light makes the crankshaft pulley seem to stand still for a moment, you will be able to read the exact position of the piston in the No. 1 cylinder on the timing scale on the front of the engine.
If you're buying a timing light, make sure the unit you select is rated for electronic or solid-state ignitions. Generally, these lights have two wires which connect to the battery with alligator clips and a third wire which connects to No. 1 plug wire. The best lights have an inductive pick-up on the third wire; this allows you to simply clip the small box over the wire. Older lights may require the removal of the plug wire and the installation of an in-line adapter. Buy quality the first time and the tool will give lasting results and ease of use.
INSPECTION AND ADJUSTMENT
See Figures 3 and 4Carbureted Engines
These engines requires a special tachometer hook-up to the service connector coming out of the distributor. As many tachometers are not compatible with this hook-up, we recommend that you consult with the manufacturer or salesman before purchasing a certain type.
NEVER allow the ignition coil terminal to become grounded; severe and expensive damage can occur to the coil and/or igniter.
- Warm the engine to normal operating temperature. Do not attempt to check timing or idle speed on a cold motor - all the readings will be different. Connect a tachometer and check the engine idle speed to be sure it is within the specification given in the Tune-Up Specifications chart or underhood emission sticker. Adjust the idle if needed and shut the engine off.
- If the timing marks are difficult to see, use a dab of paint or chalk to make them more visible.
- Connect a timing light according to the manufacturer's instructions.
- Label and disconnect the vacuum line(s) from the distributor vacuum unit. Plug it (them) with a pencil or golf tee(s).
- Be sure that the timing light wires are clear of the fan and start the engine.
- Allow the engine to run at the specified idle speed with the gearshift in correct position. Refer to the underhood emission sticker as necessary.
- Point the timing light at the marks on the tab alongside the crank pulley. With the engine at idle, timing should be at the specification given on the Tune-Up Specification Chart in this section or refer to the underhood emission sticker as necessary.
- If the timing is not at the specification, loosen the bolts at the base of the distributor just enough so that the distributor can be turned. Turn the distributor to advance or retard the timing as required. Once the proper marks are seen to align with the timing light, timing is correct.
- Stop the engine and tighten the bolts. Connect the vacuum line(s) to the distributor vacuum unit.
See Figures 5, 6 and 7
This is a general service procedure for setting base ignition timing. Refer to underhood emission sticker for any additional service procedures steps.
These engines require a special tachometer hook-up to the service connector in the diagnostic connector. As many tachometers are not compatible with this hook-up, we recommend that you consult with the manufacturer or salesman before purchasing a certain type.
NEVER allow the ignition coil terminal (tachometer terminal) to become grounded; severe and expensive damage can occur to the coil and/or igniter.
- Warm the engine to normal operating temperature. Turn off all electrical accessories. Do not attempt to check timing specification or idle speed on a cold motor - all the readings will be different.
- Connect a tachometer and check the engine idle speed to be sure it is within the specification given in the Tune-Up Specifications chart or underhood emission sticker.
- Using a small jumper wire, short both terminals of the Check Engine connector located near the wiper motor. On some engines remove the cap on the diagnostic connector. Using a small jumper wire, short terminals E1 and T together. Adjust idle speed as necessary.
- If the timing marks are difficult to see, shut engine off use a dab of paint or chalk to make them more visible.
- Connect a timing light according to the manufacturer's instructions.
- Start the engine and use the timing light to observe the timing marks. With the jumper wire in the connector the timing should be to specifications (refer to underhood emission sticker as necessary) with the engine fully warmed up (at correct idle speed) and the transmission in correct position. If the timing is not correct, loosen the bolts at the distributor just enough so that the distributor can be turned. Turn the distributor to advance or retard the timing as required. Once the proper marks are seen to align with the timing light, timing is correct.
- Without changing the position of the distributor, tighten the distributor bolt(s) and double check the timing with the light (check idle speed as necessary).
- Disconnect the jumper wire at the Check Engine connector or Diagnostic connector.
- Shut the engine off and disconnect all test equipment.