Lesson3/Learning Event 2
As the cam in the distributor turns, one of the cam lobes will contact the rubbing block on the
insulated point. If the distributor is in proper time with the engine, the contacts will open at the
time the fuel should be ignited. When the contacts in the distributor open, the primary circuit is
broken and the magnetic field collapses. The collapse of the field induces high voltage in the
secondary windings of the coil. This high-voltage current then passes to the center of the
distributor cap and to the rotor. The rotor at this time is in line with the number 1 tower on the
distributor cap. Current passes from the tower to the spark plug wire and on to the spark plug. As
the engine and distributor continue to turn, the remaining spark plugs receive the high voltage from
the coil at the proper time and in the proper sequence.
SPARK ADVANCE MECHANISMS
Spark advance mechanisms are necessary for the spark to occur in the combustion chamber at
exactly the right instant for all engine speeds. At idle, the spark is usually timed to occur at about
2 to 10 before TDC. At high speed, the spark may occur as much as 40 or more before the
piston reaches TDC on its compression stroke. It is necessary to start the fuel-air mixture burning
sooner at high speeds because the piston is traveling much faster. In fact, without a spark advance,
the piston would be well past TDC and moving downward before the burning fuel-air mixture
could create enough pressure for an effective power stroke.
There are two types of advance mechanisms in common use that will cause the distributor to
deliver a spark sooner in the cycle at high speeds. These mechanisms are the centrifugal advance
and the vacuum advance.