See Figure 1
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 controlled by turning the distributor in the engine.
Ideally, the air/fuel mixture in the cylinder will be ignited by the spark plug just as the piston passes TDC of the compression stroke. If this happens, this piston will be beginning 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 gases 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 TDC and the full benefit 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, the spark plug must fire 5 degrees before that 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. In some Jeep vehicles that were made before 1972, the advancing of the spark in the distributor was accomplished by weights alone. Others have a vacuum diaphragm to assist the weights. It is necessary to disconnect the vacuum line to the distributor when the engine is being timed.
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 engine is 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.
The timing is best checked with a timing light. This device is connected in series with the no. 1 spark plug. The current that fires the spark plug also causes the light to flash.
There is a notch on the front of the crankshaft pulley on the 4-134 engine. There are also marks to indicate TDC and 5° BTDC on the timing gear cover that will assist you in setting ignition timing.
On the 6-232 and 6-258, there is a mark on the crankshaft pulley and a scale divided into degrees. The 8-304 has the same mark and scale arrangement.
The 6-225 has the scale on the crankshaft pulley and the pointer mark on the engine.
When the engine is running, the timing light is aimed at the marks on the engine and crankshaft pulley.
There are three basic types of timing lights available. The first is a simple neon bulb with two wire connections. One wire connects to the spark plug terminal and the other plugs into the end of the spark plug wire for the No. 1 cylinder, thus connecting the light in series with the spark plug. This type of light is pretty dim and must be held very close to the timing marks to be seen. Sometimes a dark corner has to be sought out to see the flash at all. This type of light is very inexpensive. The second type operates from the car battery-two alligator clips connect to the battery terminals, while an adapter enables a third clip to be connected to the No. 1 spark plug and wire. This type is a bit more expensive, but it provides a nice bright flash that you can see even in bright sunlight. It is the type most often seen in professional shops. The third type replaces the battery power source with 110 volt current.
Connect a tachometer to the BID or SSI ignition system in the conventional way; to the negative (distributor) side of the coil and to a ground. HEI distributor caps have a Tach terminal. Some tachometers may not work with a BID, SSI, or HEI ignition system and there is a possibility that some could be damaged. Check with the manufacturer of the tachometer to make sure it can be used.
Timing should be checked at each tune-up and any time the points are adjusted or replaced. The timing marks consist of a notch on the rim of the crankshaft pulley and a graduated scale attached to the engine front (timing) cover. A stroboscopic flash (dynamic) timing light must be used, as a static light is too inaccurate for emission controlled engines.