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 (Before Top Dead Center or 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 1945-70 Jeep vehicles, 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 (After Top Dead Center or 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.
The 6-225 and 6-226 engines have the scale on the crankshaft pulley and the pointer mark on the engine.
When the engine is running, the timing light should be 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.
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.
INSPECTION AND ADJUSTMENT
See Figures 2 through 7
Some early engines utilize 6V or 24V ignition systems. Make sure your tach/dwell meter and timing light have 6V or 24V capability.
- Locate the timing marks on the pulley and on the front of the engine, or on the flywheel on CJ-2A and early CJ-3A engines.
- Clean off the timing marks so you can see them.
- Mark the timing marks with a piece of chalk or white paint. Mark the one on the engine that will indicate correct timing when it is aligned with the mark on the pulley or flywheel.
- Attach a tachometer to the engine.
- Attach a timing light according to the manufacturer's instructions. If the timing light has three wires, one is attached to the no. 1 spark plug lead with an adapter. The other two are connected to the battery. The red one goes to the positive side of the battery and the black one to the negative terminal.
- Disconnect the vacuum line to the distributor at the distributor. Plug the end of the hose.
- Check to make sure that all of the wires clear the fan and then start the engine.
- If there is an idle speed solenoid, disconnect it.
- Aim the timing light at the timing marks. If the marks that you put on the pulley and the engine are aligned, the timing is correct. Turn off the engine and remove the tachometer and the timing light. If the marks are not in alignment, proceed to the following steps.
- Turn the engine OFF .
- Loosen the distributor lock bolt just enough so that the distributor can be turned with a little effort.
- Start the engine. Keep the cords of the timing light clear of the fan.
- With the timing light aimed at the pulley and the marks on the engine, turn the distributor in the direction of rotor rotation to retard the spark, and in the opposite direction of rotor rotation to advance the spark. Line up the marks on the pulley and the engine.
- When the marks are aligned, tighten the distributor lock bolt and recheck the timing with the timing light to make sure that the distributor did not move when you tightened the distributor lockbolt.
- Turn the engine OFF the engine and remove the timing light.
On CJ-3A models beginning with engine serial No. 130859, a 41/2in. (114.3mm) starter motor was used. To use the larger starter, it was necessary to increase the width of the cylinder block flange, partially covering the flywheel hole. This makes it impossible to use the hole for timing purposes. In this event, use the timing marks on the crankshaft pulley. If a replacement block is installed with the later design in a vehicle originally equipped with the earlier design timing marks, it will be necessary to cut away enough of the flange to allow a view of the timing marks, as no other timing marks exist on these early engines.