A SPARK PLUG WIRE INSULATION TEST
checks for sparks arcing through the insulation to ground. To perform an insulation test with the hood up, block out as much light as possible, start the engine, and move a grounded screwdriver next to the insulation. If a spark jumps through the insulation to the screwdriver, the wire is bad. Spark plug leakage is a condition in which electric arcs pass through the wire insulation.

Installing new spark plug wire is a simply task, especially when one wire at a time is replaced. Wire replacement is more complicated if all of the wires have been removed. Then you must use engine firing order and cylinder numbers to route each wire correctly. Service manuals can be used to trace the wires from each distributor cap tower to the correct spark plug.

Distributor Service
The distributor is critical to the proper operation of the ignition system. The distributor senses engine speed, alters ignition timing, and distributes high voltage to the spark plugs. If any part of the distributor is faulty, engine performance suffers.

DISTRIBUTOR CAP AND ROTOR.- When problems point to possible distributor cap or rotor troubles, remove and inspect them. The distributor cap should be carefully checked to see that sparks have not been arcing from point to point. Both interior and exterior must be clean. The firing points should not be eroded, and the interior of the towers must be clean.

The rotor tip, from which the high-tension spark jumps to each distributor cap terminal, should not be worn. It also should be checked for excessive burning, carbon trace, looseness, or other damage. Any wear or irregularity will result in excessive resistance to the high-tension spark. Make sure that the rotor fits snugly on the distributor shaft.

A common problem arises when a CARBON TRACE (small line of carbonlike substance that conducts electricity) forms on the inside of the distributor cap or outer edge of the rotor. The carbon trace will short coil voltage to ground or to a wrong terminal lug in the distributor cap. A carbon trace will cause the spark plugs to either fire poorly or not at all.

Using a droplight, check the inside of the distributor cap for cracks and carbon trace. Carbon trace is black which makes it hard to see on a black-colored distributor cap. If carbon trace or a crack is found, replace the distributor cap or rotor.

CONTACT POINT DISTRIBUTOR SERVICE.- In a contact point distributor, there are two areas of concern- the contact points and the condenser.

Bad contact points cause a variety of engine performance problems. These problems include high-speed missing, no-start problem, and many other ignition troubles. Visually inspect the surfaces of the contact points to determine their condition. Points with burned and pitted contacts or with a worn rubbing block must be replaced. However, if the points look good, point resistance should be measured. Turn the engine over until the points are closed and then use an ohmmeter to connect the meter to the primary point lead and to ground. If resistance reading is too high, the points are burned and must be replaced.

A faulty condenser may leak (allow some dc current to flow to ground), be shorted (direct electrical connection to ground), or be opened (broken lead wire to the condenser foils). If the condenser is leaking or open, it will cause point arcing and burning. If the condenser is shorted, primary current will flow to ground and the engine will NOT start. To test a condenser using an ohmmeter, connect the meter to the condenser and to ground. The meter should register slightly and then return to infinity (maximum resistance). Any continuous reading other than infinity indicates that the condenser is leaking and must be replaced.

Installing contact points is a relatively simple procedure but must be done with precision and care in order to achieve good engine performance and economy. Make sure the points are clean and free of any foreign material.

Proper alignment of the contact points is extremely important (fig. 2-52). If the faces of the contact points do not touch each other fully, heat generated by the primary current cannot be dissipated and rapid burning takes place. The contacts are aligned by bending the stationary contact bracket only. NEVER BEND THE MOVABLE CONTACT ARM. Ensure the contact arm-rubbing block rests flush against the distributor cam. A small amount of an approved lubricate should be placed on the distributor cam to reduce friction between the cam and rubbing block. Once the points are installed, they can be adjusted using either a feeler gauge or dwell meter.

To use a feeler gauge to set the contact points, turn the engine over until the points are FULLY OPEN. The rubbing block should be on top of a distributor cam lobe. With the points open, slide the specified thickness feeler gauge between them. Adjust the points so that there is a slight drag on the blade of the feeler gauge. Depending upon point design, use a screwdriver or Allen wrench to open and close the points. Tighten the hold-down screws and recheck the point gap. Typically point gap settings average around .015 inch for eight-cylinder engines and .025 inch for six-and four-cylinder engines. For the gap set of the engine you are working on, consult the manufacturer's service manual.

Figure 2-52.- Contact point alignment.

CAUTION
Ensure the feeler gauge is clean before inserting it between the points. Oil and grease will reduce the service life of the points.

To use a dwell meter for adjusting contact points, connect the red lead of the dwell meter to the distributor side of the ignition coil (wire going to the contact points). Connect the black lead to ground.

If the distributor cap has an adjustment window, the points should be set with the engine running. With the meter controls set properly, adjust the points through the window of the distributor cap using a Allen wrench or a special screwdriver. Turn the point adjustment screw until the dwell meter reads within manufacturer's specification. However, if the distributor cap does not have an adjustment window, remove the distributor cap and ground the ignition coil wire. Then crank the engine; this action will simulate engine operation and allow point adjustment with the dwell meter.

Dwell specifications vary with the number of cylinders. An eight-cylinder engine requires 30 degrees of dwell. An engine with few cylinders requires more dwell time. Always consult the manufacturer's service manual for exact dwell values.

Dwell should remain constant as engine speed increases or decreases. However, if the distributor is worn, you can have a change in the dwell meter reading. This is known as DWELL VARIATION. If dwell varies more than 3 degrees, the distributor should either be replaced or rebuilt. Also, a change in the point gap or dwell will change ignition timing. For this reason, the points should always be adjusted before ignition timing.