A distributor is a combination of two mechanically activated switches in a plastic and metal housing. One of the switches is a simple on and off switch. This is called the points.
The other switch is a rotary switch that rotates to as many contacts as you have cylinders in your engine. The moving part of the switch is called the rotor. The switch contacts are in the distributor cap. The rotor sits on top of the distributor shaft and is rotated by the shaft so that it touches each contact in succession.
Just below the rotor is a multi-lobed cam that opens and closes the points. There are as many lobes on the cam as there are cylinders in the engine.
There are also distributers that operate without moving partsthat operate on a photo-cell principle. However, all distibuters have the same aim,. to deliver a good spark from the coil at precisely the right moment to the right cylindar. Combustion powers the car. Good combustion is made up of air, fuel, compression and a spark. The distributer delivers the spark and should do it well.
The points have two contacts. One contact is stationary. The other contact has a plastic follower on its arm that rides on the distributor shaft cam. The cam pushes the follower, opening and closing the points.
One of the points is grounded, providing a return path to the battery. The other point carries electricity from the low voltage side of the coil. When the points are open, high voltage flows from the coil, through the center contact in the distributor cap, through the rotor, the cap, the spark plug and to the ground.
When you set the point gap, you are setting up the length of time the switch will be on & off in relation to the turning of the distributor shaft.
The center contact is a spring loaded carbon contact that rides on the center of the rotor. The Rotor is just long enough to reach the outer contacts on the distributor cap. The high voltage electricity from the coil goes through the big wire coming out the end of the coil to the center of the distributor cap. It then travels down the carbon contact to the metal plate on the rotor, along the metal plate to one of the outer contacts on the top of the distributor cap. It goes out of the distributor, through the spark plug wire to the spark plug. A spark jumps the gap in the spark plug igniting the air fuel mixture causing an explosion in the cylinder.
The important thing is that the rotor needs to be pointing at the right terminal when the jolt of electricity is coming from the coil. This is set by rotating the distributor body.
Once the plastic follower is sitting at the tip of the cam lobe, you can adjust the point gap. On most British cars the gap is set to 16 thousandths of an inch. You slightly loosen the screw holding the points down and move the arm until the gap is correct.
This means the 16 thousandths feeler gauge will go between the points without opening them farther, but a 17 thousandths feeler gauge will not. Tighten the mounting screw. You are now ready to adjust the timing.
A continuity light can be purchased at any auto parts store or you can make one yourself. It is basically a light with two wires attached. You connect one wire to ground and the other to the movable arm of the points. Turn the ignition switch to the on position. When the points are open, the light is lit. When the points are closed, the light is not lit.
First, locate the number one spark plug. For inline engines, it is the one closest to the radiator. Follow the number one plug's wire back to the rotor. When the number one plug fires, the rotor will be pointing to that contact . Remember where the contact is located along the circumference of the distributor then remove the cap so you can see where the rotor is pointing.
Line up the timing marks with the rotor pointing at the cap terminal that has the spark plug wire going to the number one spark plug. Since the crank rotates twice as fast as the distributor shaft you have a 50% chance of getting it right the first time. If the rotor is pointing to a different terminal, rotate the engine one more time until the timing marks are lined up and the rotor is pointing to the number one plug.
What you want is for the points to just be opening when the timing point is lined up. This is where the continuity light just comes on.
If you look at the plastic follower that rubs against the distributor cam, you want it to be on the leading side of a cam lobe. This will be the counter clockwise, or left side of the cam lobe.
Attach the continuity light and turn the ignition switch on. With the follower on the correct side of the lobe, slowly turn the distributor clockwise until the continuity light just comes on (The points just start to open). If it's already on, move the distributor body counter clockwise (retard) until the continuity light goes off. Then slowly rotate it clockwise.
Once you are satisfied that you have the distributor body to the point where the points are just opening (the light just goes on), turn the ignition off and tighten the base of the distributor. Reassemble everything and go out for a drive.