See Figure 1
The points function as a circuit breaker for the primary circuit of the ignition system. The ignition coil must boost the 12 volts of electrical pressure supplied by the battery to as much as 25,000 volts in order to fire the plugs. To do this, the coil depends on the points and the condenser to make a clean break in the primary circuit.
The coil has both primary and secondary circuits. When the ignition is turned ON , the battery voltage is directed through the coil and to the points. The points are connected to ground, completing the primary circuit. As the current passes through the coil, a magnetic field is created in the iron center core of the coil. When the cam in the distributor turns, the points open, breaking the primary circuit. The magnetic field in the primary circuit of the coil then collapses and cuts through the secondary circuit windings around the iron core. Because of the physical principle called electromagnetic induction, the battery voltage is increased to a level sufficient to fire the spark plugs.
When the points open, the electrical charge in the primary circuit tries to jump the gap created between the open contacts of the points. If this electrical charge were not transferred elsewhere, the metal contacts of the points would start to change rapidly.
The function of the condenser is to absorb excessive voltage from the points when they open and thus prevent the points from becoming pitted or burned.
- Disconnect the high tension wire from the top of the distributor and the coil.
- The distributor cap is retained by two spring clips. Insert a screwdriver under their ends and release them. Lift off the cap with the spark plug wires attached. Inspect the inside of the cap. Wipe it clean (with a rag) and check for burned contacts, cracks and carbon tracks. A carbon track shows as a dark line running from one terminal to another. It cannot be successfully removed, so replace the cap if it has one or more of these. Generally, a cap and rotor will last 36,000 miles (58,000 km).
- Remove the rotor from the distributor shaft by pulling it straight up. Examine the condition of the rotor. If it is cracked or the metal tip is excessively worn or burned, it should be replaced. Clean the metal tip with a clean cloth but don't file it.
- Pry open the contacts of the points with a screwdriver and check the condition of the contacts. If they are excessively worn, burned or pitted, they should be replaced.
- If the points are in good condition, adjust them, then install the rotor and the distributor cap. If the points need to be replaced, follow the replacement procedure given next.
REMOVAL & INSTALLATION
See Figure 2
- Remove the cap and rotor as outlined in Steps 1-3 of the preceding section.
- On single point distributors, loosen the two screws securing the points. Use a magnetic screwdriver to avoid losing a screw down the distributor. Loosen the point wire screw on the side of the distributor and slip the wire out. Remove the point set.
- On dual point distributors, decide if the tools available to you will enable you to remove the two condensers on the outside of the distributor body. If you have a flexible screwdriver or one bent at a 90° angle, you should be able to get at them. Otherwise, it may be best to remove the distributor from the engine for points and condenser replacement.
To remove the distributor, perform the following procedure:
- Using a scribing tool, mark the distributor-to-engine position on the hold-down plate at the front of the distributor.
- Mark the position of the rotor in relation to the distributor body. Do this by simply replacing the rotor on the distributor shaft and marking the spot on the distributor body where the rotor is pointing.
- Remove the small bolt at the rear of the distributor and lift the distributor from the engine. DO NOT CRANK THE ENGINE WITH THE DISTRIBUTOR REMOVED.
- On dual point distributors, loosen the two mounting screws which secure the points. DO NOT loosen the factory preset phase adjusting screw. Loosen the screws retaining the points' wires and remove each point set. The bottom of the point set is slotted; thus, it is not necessary to completely remove the points' mounting screws.
- On single point distributors, the condenser is mounted on the side of the distributor. Disconnect the wire and remove the condenser.
- On dual point distributors, the two condensers are mounted on the outside of the distributor body. Note that the two are different. When new condensers are installed, they must be replaced in the same relationship as the originals. This is important, for they have different electrical capacities. Remove the condenser mounting screws and loosen the condenser lead screws, then remove the condensers.
- Before installing the new points and condenser(s), place a matchhead sized dab of grease on the distributor shaft cam and smear it evenly around the cam.
- Install the new point set(s) and condenser(s). Be sure the dual point condensers are positioned in their proper locations. The different mounting ears on each make them pretty much foolproof. Tighten the condenser mounting screws but leave the points screws slightly loose.
- Check that the faces of the points meet squarely. If not, the fixed mount can be bent slightly with gentle force, using a pair of needlenose pliers. DO NOT bend the movable contact.
- The point gap must be adjusted next. The gap is adjusted with the rubbing block of the points resting on one of the six high spots of the distributor cam. To get it there, the engine can be rotated by bumping the starter with the ignition key or turning the crankshaft, with a wrench on the crankshaft pulley bolt; this is easier to do with the spark plugs removed.
If the distributor is removed (dual points), the distributor shaft can be rotated until the rubbing blocks are resting on the high spots of the cam. It won't matter if you move the distributor shaft; it can only go back into the engine one way. Just note the position from which it is moved and move it back there, prior to replacing the distributor in the engine.
- Insert a 0.5mm thick flat feeler gauge between the points. A slight drag should be felt. If no drag can be felt or if the gauge cannot be inserted at all, insert a screwdriver into the eccentric adjusting screw or into the notch provided for adjustment, and use it to open or close the gap between the points until it is correct.
- When the gap is set, tighten the points' screws and recheck the gap. Sometimes it takes 3-4 tries to get it correct, so don't feel frustrated if they seem to move around on you a little. It is not easy to feel the correct gap, either. Use gauges 0.05mm larger or smaller than the 0.5mm as a test. If the points are spread slightly by a 0.5mm gauge and not touched at all by a 0.4mm gauge, the setting should be right.
- After the adjustment is completed, pull a clean piece of tissue or a white business card between the points to clear any bits of grit.
- On dual point distributors, if the distributor was removed, reset the shaft and rotor to their original positions, then install the distributor. Note that the slot for the oil pump drive is tapered and will only fit one way. Be sure the marks you made earlier line up and tighten the distributor hold-down bolt.
- Replace the rotor and distributor cap, then snap on the clips. If you have a dwell meter (recommended) you should next set the dwell. Otherwise, go on to the ignition timing.
Single Point Distributor
The dwell angle is the number of degrees of distributor cam rotation through which the points remain closed (conducting electricity). Increasing the point gap decreases the dwell, while decreasing the gap increases the dwell.
The dwell angle may be checked with the distributor cap and rotor installed while the engine is running or with the cap and rotor removed while the engine is cranking at starter speed. The meter gives a constant reading with the engine running. With the engine cranking, the meter will fluctuate between 0° dwell and the maximum figure for that setting. Never attempt to adjust the points when the ignition is ON or you may receive a shock.
- Connect a meter as per the manufacturer's instructions (usually one lead to the distributor's terminal of the coil and the other lead to a ground). Zero the meter, if necessary.
- Check the dwell by either the cranking method or with the engine running. If the setting is incorrect, the points must be adjusted.
- To change the dwell angle, turn the ignition OFF , loosen the points hold-down screw and adjust the point gap; increase the gap to decrease the dwell and vice-versa. Tighten the hold-down screw and check the dwell angle with the engine cranking. If it seems to be correct, replace the cap and rotor, then check the dwell with the engine running. Readjust as necessary.
- Run the engine speed up to about 2,500 rpm and let the speed drop abruptly; the dwell reading should not change. If it does, a worn distributor shaft, bushing, cam or breaker plate is indicated. The parts must be inspected and replaced, if necessary.
- After adjusting the dwell angle, check/adjust the Ignition Timing. Ignition timing must be checked after adjusting the point gap, as a 1° increase in dwell results in an ignition timing retard of 2° and vice-versa.
Adjust the point gap of a dual point distributor with a dwell meter as follows:
- Disconnect the wiring harness of the distributor from the engine wiring harness.
- Using a jumper wire (a length of wire with an alligator clip at each end), connect the black wire of the harness (engine side) to the black wire of the distributor side of the harness (advance points).
- Start the engine and observe the reading on the dwell meter. Turn the engine OFF and adjust the points according to the single point distributor procedure.
- Disconnect the jumper wire from the black wire of the wiring harness (distributor side) and connect it to the yellow wire (retard points).
- Adjust the point gap as necessary, in the same manner.
- After the dwell of both sets of points is correct, remove the jumper wire and connect the engine-to-distributor wiring harness securely.