Ignition Timing

GENERAL INFORMATION

Ignition timing is an important part of the tune-up. It should be checked to compensate for timing belt or gear wear at the interval specified in the Maintenance Chart. An inductive type DC light, one that can be used on electronic ignition, and powered by the vehicle's battery, is the most frequently used by professional tuners. The bright flash put out by the DC light makes the timing marks stand out on even the brightest of days. The DC light attaches to the spark plug and the wire with an adapter and two clips attached to the battery posts for power.

CAUTION
When performing this or any other operation with the engine running, be very careful of the alternator belt and pulleys. Make sure that your timing light wires don't interfere with the belt.

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 will be ignited by the spark plug just before the piston passes TDC of the compression stroke. If this happens, the piston will be beginning its downward motion of the power stroke just as the compressed and ignited air/fuel mixture begins to develop a considerable amount of pressure.The expansion of the air/fuel mixture then forces the piston down on the power stroke and turns the crankshaft. Because it takes time for the mixture to burn, the spark plug must fire a little before the piston reaches TDC. Otherwise, the mixture will not be burned completely early enough in the downstroke 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 (5B), 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 a 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. It is necessary to disconnect the vacuum line from the diaphragm 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 there will be excessive temperature and pressure. 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 and creates less pressure in the cylinder, resulting in poor engine performance and lack of power. The timing is best checked with a timing light.This device is connected in series (or through induction) with the No. 1 spark plug. The current which fires the spark plug also causes the timing light to flash. The timing marks are located at the front crankshaft pulley and consist of a notch on the crankshaft pulley and a scale of degrees of crankshaft rotation attached to the front cover. When the engine is running, the timing light is aimed at the marks on the flywheel pulley and the pointer.