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.
Points (the on-off switch)
The points are a replaceable on and off switch located on a plate
within the top of your distributor. The points control when the
coil will send a jolt of electricity through the cap and rotor
to a spark plug. The spark occurs as the points just open.
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
The stationary point is grounded, providing a return path to
the battery. The moving 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.
Cap & rotor (the rotary switch)
The rotor sits on top of the distributor shaft and rotates around
in circles. It has a metal strip that goes from the center of the
rotor top to the outside of the rotor cap. This is a switch contact.
The cap has a center contact and as many contacts around the top
edge of the cap as there are cylinders in the engine.
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.
To set the point gap:
Locate the plastic cam follower on the movable point arm. To set
the timing, this follower needs to be sitting at the tip of a
cam lobe. If you are going to set the timing afterwards, you can
loosen the ring at the base of the distributor and rotate the
distributor until the plastic follower is sitting on the tip of
a cam lobe. If you are not planing to adjust the timing afterward,
put the car into fourth gear and push the car forward until the
plastic follower is sitting on the tip of the cam lobe. This is
best done with a helper and in a place where the car will not
get away from you. Alternatively, you can put the transmission
into neutral and turn the engine over with the starter crank or
use the starter handle to turn the engine.
Once the plastic follower is sitting at the tip of the cam lobe,
you can adjust the point gap. On most British cars, including
Series Land Rovers,the gap is set to 15 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 15 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 is used to adjust the timing when the engine
is not on. The process is called stationary 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.
Adjusting the timing:
The distributor shaft rotates in a counter clockwise direction.
If you turn the distributor housing counter clockwise, the direction
that the rotor is turning, you are retarding the timing. If you
turn the distributor clockwise, into the rotation of the rotor you
are advancing the timing.
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
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 right side of the cam lobe. When
finding the point where the points just open it is important that
the distributor shaft be turning in the direction it normally travels
when the engine is running (
turn the distributor clockwise not
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. If you go past that point, back
the distributor up then try it again by turning the distributor
clockwise. The timing will be off if you try to reach the
place where the light just comes on by backing up.
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.
There is now an FAQ page on trouble
shooting the ignition system. It goes into the mechanical and
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