Unless the trouble is known, a systematic procedure should be followed to locate the cause. Remember, that electric current will follow the path of least resistance. Trace ignition wiring while checking for grounds, shorts, and open circuits. Bare wires, loose connections, and corrosion are found through visual inspection.

After checking the system, you must evaluate the symptoms and narrow down the possible causes. Use your knowledge of system operation, a service manual troubleshooting chart, basic testing methods, and common sense to locate the trouble. Many shops have specialized equipment that provide the mechanic a quick and easy means of diagnosing ignition system malfunctions.

Spark Plugs and Spark Plug Wires
Bad spark plugs cause a wide range of problems- misfiring, lack of power, poor fuel economy, and hard starting. After prolonged use, the spark plug tip can become coated with ash, oil, and other residue. The spark plug electrodes can also bum and widen the gap. This makes it more difficult for the ignition system to produce an electric arc between the electrodes.

To read spark plugs closely, inspect and analyze the condition of each spark plug tip and insulator. This will give you information on the condition of the engine, the fuel system, and the ignition system. The conditions commonly encountered with spark plugs areas follows:

NORMAL OPERATION (fig. 2-47) appears as brown to grayish-tan deposit with slight electrode wear. This indicates the correct spark plug heat range and mixed periods of high-and low-speed operation. Spark plugs, having this appearance, may be cleaned, regapped, and reinstalled.

CARBON FOULED (fig. 2-48) appears as dry, fluffy black carbon, resulting from slow operating speeds, wrong heat range (too cold), weak ignition (weak coil, worn ignition cables, etc.), faulty automatic choke, sticking manifold control valve, or rich air-fuel mixture. Spark plugs, having this appearance, may be cleaned, regapped, and reinstalled.

OIL FOULED (fig. 2-49) appears as wet, oily deposits with very little electrode wear, resulting from worn rings, scored cylinder, or leaking valve seals. Spark plugs, having this appearance, may be degreased, cleaned, regapped, and reinstalled.

Figure 2-47.- Normal operation.

Figure 2-48.- Carbon fouled.

Figure 2-49.- Oil fouled.

ASH FOULED (fig. 2-50) appears as red, brown, yellow, or white colored deposits which accumulate on the insulator, resulting from poor fuel quality or oil entering the cylinder. Most ash deposits have no adverse effect on the operation of the spark plug as long as they remain in a powdery state. However, under certain conditions these deposits melt and form a shiny glaze on the insulator which, when hot, acts as a good electrical conductor. This allows current to follow the deposit instead ofjumping the gap, thus shorting out the spark plug. Spark plugs, having a powdery condition, may be cleaned, regapped, and replaced. Those having a glazed deposit are to be replaced.

PREIGNITON DAMAGE (fig. 2-51) appears as burned or blistered insulator tips and badly worn

Figure 2-50.- Ash fouled.

electrodes, resulting from over-advanced timing, low-octane fuel, wrong spark plug heat range (too high), or a lean air-fuel mixture. Spark plugs, having this condition, are to be replaced with ones having the recommended heat range.

When a spark plug is removed for cleaning or inspection, it should be regapped by the engine manufacturer's specifications. New spark plugs are also to be regapped before installation, as they may have been dropped or mishandled and are not within specifications.

A wire type feeler gauge should be used to measure spark plug gap. Slide the feeler gauge between the electrodes. If needed, bend the side electrode until the feeler gauge fits snugly. The gauge should drag

Figure 2-51.- Preignition damage.

slightly, as it is pulled in and out of the gap. Spark plug gaps vary from 0.30 inch on contact point ignitions to over 0.60 inch on electronic ignition systems.

When the spark plugs are being reinstalled, tighten them to the manufacturer's recommendation. Some manufacturers give spark plug torque, while others recommend bottoming the plugs on the seat and then turning an additional one-quarter to one-half turn. Refer to the manufacturer's service manual for exact procedures.

A faulty spark wire can either have a burned or broken conductor, or it could have deteriorated insulation. Most spark plugs wires have a resistance conductor that can be easily separated. If the conductor is broken, voltage and current cannot reach the spark plug. If the insulation is faulty, sparks may leak through to ground or to another wire instead of reaching the spark plugs. To test the wires for proper operation, you can perform the following: