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10-58
IGNITION SYSTEM
•
ELECTRICAL SYSTEMS
(334)
Figure 10-6 6 -Centrlfugal Advance
Mechanism
after the spark has occurred at the spark plug
gap.
The sequence of action described above is re-
peated
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.
2. Centrifugal advance
which is governed
by speed of engine. The centrifugal advance
mechanism built into the distributor consists of
an advance cam which is integral with the dis-
tributor cam, a pair of advance weights, two
springs, and a weight base plate which is in-
tegral with the distributor shaft. At low speeds
the springs hold the advance weights as shown
in figure 10-66, view A, so that there is no ad-
ditional spark advance and the spark occurs in
accordance with the initial manual setting of
distributor. As speed increases, centrifugal
force causes the advance weights to throw out-
ward and push the advance cam, thus rotating
the distributor cam ahead of the distributor
shaft. This causes the distributor cam lobes to
open and close the contact points earlier in the
compression stroke so that the spark is ad-
vanced. See figure 10-66, view B.
Figure 10-67-Vacuum Advance Mechanism
3.
Vacuum Controlled Advance
which is
governed by manifold vacuum during part
throttle operation. The vacuum control mount-
ed on the side of distributor housing contains
an air-tight, spring loaded diaphragm which is
connected by a link to the breaker plate. The
breaker plate is mounted so that the plate and
attached contact points can be rotated around
the distributor cam by the control link. See
figure 10-67. The spring-loaded side of the dia-
phragm is connected by a pipe to an opening in
the carburetor barrel. This opening is on the
atmospheric side of the throttle valve when the
throttle is in idling position so that there will
be no vacuum to operate the advance mechan-
ism. When the throttle valve is opened equiva-
lent
to approximately 18 MPH, the vacuum at
the opening is sufficient to act on the control
diaphragm, causing it to compress the spring
B-FULL ADVANCE
SPRIN G
PLATE
A-NO
ADVANCE
ADVA
NC E
WEIG
HTS
c.
Control of Spark Timing
The timing of the spark with respect to pis-
ton position in the cylinder must vary in
accordance with operating conditions if best
engine performance is obtained. The spark ad-
vance 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 pis-
t
on
as it starts down on the compression
stroke.
Under part throttle light load operation,
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 acceleration or
on
heavy
loads (wide open throttle) the spark
advance
required to develop the maximum
power of the engine is considerably less than
that required for light loads.
Control
.
of spark timing to satisfy these
constantly changing operating requirements is
obtained in three ways, as follows
:
1.
Initial, manual setting of distributor
which is made so that contact points open at a
specified position of piston, as indicated by a
timing mark on flywheel. See Ignition Timing
(par. 10-47).
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