GMC 1976 ZEO 6083 Maintenance Manual page 411

The cap, rotor, and high voltage leads in a dis-
tributor form a distribution system that conveys the
high voltage surges to the spark plugs in correct
sequence .
The breaker plate contains the breaker lever, con-
tact support, and capacitor . When the breaker cam
rotates, each cam lobe passes by and contacts the
breaker lever rubbing block, separating the contact
points and producing a high voltage surge in the
ignition system . With every breaker cam revolution,
one spark will be produced for each engine cylinder .
Since each cylinder fires every other revolution in a
four-cycle engine, the distributor rotates at one-half
engine speed.
The shaft and weight base assembly is fitted in
suitable bearings made of such materials as cast iron,
bronze, or iron . Centrifugal advance weights are pi
voted on studs in the weight base, and are free to
move against calibrated weight springs which con-
nect them to the breaker cam assembly . The breaker
cam assembly fits on the top of the shaft (slip fit) and
rotates with the shaft, being driven by the weight
springs actuated by the advance weights.
Outward movement of the weights advances the
cam assembly in relation to the shaft as engine speed
is increased, providing an earlier spark.
It is possible to improve fuel economy on engines
operating under part-throttle conditions by supply-
ing additional spark advance. Vacuum advance
mechanisms are provided on distriubtors for this
purpose. The mechanism used rotates the breaker
plate in order to time the spark earlier when the
engine is operating at part throttle .
Centrifugal Advance
The centrifugal advance mechanism times the
high voltage surge produced by the ignition coil so
that it is delivered to the engine at the correct instant,
as determined by engine speed.
When the engine is idling, the spark is timed to
occur in the cylinder just before the piston reaches
top dead center . At higher engine speeds, however,
there is a shorter interval of time available for the
fuel-air mixture to ignite, burn, and give up its power
to the piston . Consequently, in order to obtain the
maximum amount of power from the mixture, it is
necessary at higher engine speeds for the ignition
system to deliver the high voltage surge to the cylin-
der earlier in the cycle.
To illustrate this principle, assume that the burn-
ing time of a given gas mixture in an automotive
engine is .003 of a second . To obtain full power from
combustion, maximum pressure must be reached
ENGINE ELECTRICAL
6Y- 3 1
while the piston is between 10 degrees and 20 degrees
past top dead center . At 1,000 engine rpm, the crank-
shaft travels through 18 degrees in .003 of a second,
at 2,000 rpm, the crankshaft travels through 36 de-
grees. Since maximum pressure point is fixed, it is
easy to see why the spark must be delivered into the
cylinder earlier in the cycle in order to deliver full
power, as engine speed increases.
As previously mentioned, the timing of the spark
to engine speed is accomplished by the centrifugal
advance mechanism, which is assembled on the dis
tributor shaft. The mechanism, consists primarily of
two weights and a cam assembly . The weights throw
out against spring tension as engine speed increases.
This motion of the weights turns the cam assembly
so that the breaker cam is rotated in the direction of
shaft rotation to advanced position with respect to
the distributor drive shaft. The higher the engine
speed, the more the weights throw out and the fur-
ther the breaker cam is advanced .
The centrifugal advance required varies consider-
ably between various engine models . In order to de-
termine 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. Tim-
ing, 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 . Un-
der these conditions, additional spark advance (over
and above advance provided by the centrifugal ad-
vance mechanism) will 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 con-
nected 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 . 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 cham-