Chevrolet Light Duty Truck 1973, Service Manual

Page: 558 / 979

6 Y -3 0 EN G INE ELECTRICAL
1,000 ENG. RPM
2,000 ENG. RPM
Fig. 5 i— Ig n itio n S p a r k vs E n g in e S pe e d
higher the engine speed, the more the weights throw out
and the further the breaker cam is advanced. See Fig. 6i.
The centrifugal advance required varies considerably
between various engine models. In order to determine
the advance for a given engine, the engine is operated
on a dynamometer at various speeds with a wide-open
throttle. Spark advance is varied at each speed until the
range of advance that gives maximum power is found.
The cam assembly, weights and springs are then selected
to give this advance. Timing, consequently, varies from
no advance at idle to full advance at high engine speed
where the weights reach the outer limits of their travel.
Vacuum Advance
Under part-throttle operation a high vacuum develops in
the intake manifold and a smaller amount of air and
gasoline enters the cylinder. Under these conditions,
additional spark advance (over and above advance
provided by the centrifugal advance mechanism) will
Fig. 6i—Centrifugal Advance Mechanism
increase fuel economy. In order to realize maximum
power, ignition must take place still earlier in the cycle.
To provide a spark advance based on intake manifold
vacuum conditions, many distributors are equipped with
a vacuum advance mechanism. The mechanism has a
spring-loaded diaphragm connected by linkage to the
distributor. The spring-loaded side of the diaphragm is
air-tight, and is connected in many cases by a vacuum
passage to an opening in the carburetor. See Fig.
l i .
This
opening is on the atmospheric side of the throttle when
the throttle is in the idling position. In this position,
there is no vacuum in the passage.
When the throttle is partly opened, it swings past the
opening of the vacuum passage. Intake manifold vacuum
then can draw air from the air-tight chamber in the
vacuum advance mechanism and this causes the
diaphragm to be moved against the spring. This motion
is transmitted by linkage to the distributor breaker
assembly rotation is governed by the amount of vacuum
in the intake manifold up to the limit imposed by the
design of the vacuum advance mechanism.
When the distributor breaker plate assembly is rotated,
the contact points are carried around the breaker cam to
an advanced position, so that the breaker cam contacts
the rubbing block and closes and opens the points earlier
in the cycle. This provides a spark advance based on the
amount of vacuum in the intake manifold. Thus, for
varying compressions in the cylinder the spark advance
will vary, permitting greater economy of engine
operation. It should be recognized that the additional
advance provided by vacuum control is effective in
providing additional economy only on PART-THROT-
TLE operation.
At any particular engine speed there will be a certain
definite advance resulting from operation of the
LIGHT DUTY TRUCK SERVICE MANUAL