Buick LeSabre 1965 Service Manual page 605

ELECTRICAL SYSTEMS
IGNITION SYSTEM 1 0 - 3 5
small fraction of a second. Dur-
ing this interval the condenser
discharges back through the pri-
mary circuit, producing an os-
cillation of the current flow in the
primary circuit during the brief
instant that is required for the
primary circuit to return to a
state of equilibrium. Note par-
ticularly that the ignition con-
denser does not discharge until
after the spark has occurred at
the spark plug gap.
The sequence of action described
above is repeated as each lobe of
the distributor cam moves under
and past the rubbing block on the
contact breaker arm to cause the
contact points to close and open.
d. Control of Spark Timing
The timing of the spark with res-
pect to piston position in the cyl-
inder must vary in accordance
with operating conditions if best
engine performance is obtained.
The spark advance for obtaining
satisfactory idling should be as
low as possible. At high speed,
the spark must occur earlier in
the compression stroke in order
to give the fuel-air mixture ample
time to ignite, burn and deliver
its power to the piston as it starts
down on the power stroke.
Under part throttle light load op-
eration, a smaller amount of fuel-
air mixture (by weight) enters the
cylinder so that the mixture is
less highly compressed. Under
this condition, advancing the spark
permits fuller utilization of the
fuel-air charge. During accelera-
tion or on heavy loads (wide open
throttle) the spark advance re-
quired to develop the maximum
power of the engine is consider-
ably less than that required for
light loads.
Control of spark timing to satisfy
these constantly changing operat-
ing requirements is obtained in
three ways, as follows:
1. Initial, manual setting of dis-
tributor is made so that contact
points open at a specified position
of piston, as indicated by a timing
mark on crankshaft balancer. See
Ignition Timing (par. 10-35)
2. Centrifugal Advance is gov-
erned by engine speed. The cen-
trifugal advance mechanism is
located above the circuit breaker
cam inside the rotor and consists
of an advance cam which is in-
tegral with the distributor shaft,
a pair of advance weights, two
springs, and a weight base plate
which is assembled to the dis-
tributor cam.
At idle speeds, the springs hold
the advance weights as shown in
Figure 10-35A, so that there is
no spark advance and the spark
occurs in a c c o r d a n c e w i t h
the initial m a n u a l setting of
distributor.
As speed increases, centrifugal
force causes the advance weights
to throw outward and push against
the advance cam, thus rotating the
weight base plate and integral
distributor cam ahead of the dis-
tributor shaft. This causes the
distributor cam lobes to open and
close the contact points earlier
in the compression stroke so that
the spark is advanced. See Figure
10-35B.
3. Vacuum Advance is governed
by manifold vacuum. The contact
point set is mounted on a breaker
plate which is located below the
centrifugal advance mechanism
and uses the outer diameter of
the upper shaft bushing for its
bearing surface. The movable
breaker plate is held in position
on the upper shaft bushing by a
retainer. The vacuum unit is
mounted on the distributor base
and under the movable breaker
plate so that the breaker plate
can be rotated around the distri-
butor cam by a link from the vac-
uum unit. See Figure 10-36.
The vacuum u n i t contains a
spring - loaded diaphragm. The
spring-loaded side of the dia-
phragm is connected by a hose to
an opening in the carburetor bar-
rel. This opening may be above
or below the edge of the throttle
valve when the engine is idling,
depending on carburetor design.
If the opening is below the throttle
valve, the distributor will always
have vacuum advance, even at
idle; if the opening is above the
throttle valve at idle, the vacuum
advance unit will not begin to
operate u n t i l the throttle i s
opened slightly, causing the edge
ADVANCE
WEIGHTS
ADVANCE
"CAM
A - N O ADVANCE
ADVANCE
WEIGHT
BASE
B—FULL ADVANCE
Figure 10-35—Centrifugal Advance Mechanism