Chevrolet 70 1969 Series Shop Manual page 386

N O T E : Cam angle readings taken at speeds
above 1750 engine rpm may prove unreliable on
some cam angle meters.
CENTRIFUGAL ADVANCE
Distributor is equipped with centrifugal spark
advance mechanism. Where speed variations are
encountered, spark advance, based on engine speed,
is necessary to develop maximum power. Centrif­
ugal advance mechanism is located under the break­
er plate assembly, and is a part of the shaft as­
sembly. It consists of an automatic cam actuated
by two centrifugal weights controlled by springs.
As engine speed increases, the weights grad­
ually throw out and rotate the cam to provide the
desired spark advance for speed at which engine
is running. The correct weights, cam contour, and
spring calibration have been selected to provide
spark advance which will give the bestperformance
of engine throughout its entire speed range. Cen­
trifugal advance mechanism can be checked for
freeness of operation as previously directed under
"Ignition Maintenance." A distributor tester must
be used to check advance action under various
speed conditions.
VACUUM ADVANCE
The vacuum advance control unit is mounted
to the base of the distributor housing. The vacuum
control unit consists of an enclosed, calibrated,
spring-loaded diaphragm and is linked to the mov­
able breaker plate. Under part throttle operation,
the intake manifold vacuum is sufficient to actuate
the vacuum control diaphragm and cause the break­
er plate to move, advancing the spark and increas­
ing fuel economy. During acceleration or when
engine is under heavy load, the vacuum is not suf­
ficient to actuate the diaphragm and the breaker
plate is held in the retarded position by a calibrat­
ed return spring which bears against the vacuum
diaphragm.
DISTRIBUTOR REMOVAL
1. Locate No. 1 cylinder spark plug wire on
distributor cap; mark this position on cap and
mark distributor housing adjacent to No. 1 wire
in cap.
2. Release distributor cap hold down screws;
then remove cap and place clear of work area.
3. Disconnect the distributor primary wire
from terminal on ignition coil.
4. Intermittently operate
crankshaft pulley comes to rest with the 10
starter until the
BUDC
mark aligned with pointer (fig. 2). With pointer
and timing mark aligned, rotor segment should
point toward mark made on distributor body in
Step 1 above. Instead, rotor segment may point
180 degrees away from mark; in this case, rotate
engine one complete revolution and realign timing
mark with pointer.
N O T E : Distributor can be removed and readily
reinstalled if engine remains in this position.
5. Disconnect vacuum line from distributor,
then remove cap screw and mountingclamp (fig. 3).
Lift distributor straight up until spiral gear dis­
engages camshaft gear. Rotor will rotate a few
degrees clockwise as gears disengage. Mark this
position of rotor on distributor housing, also note
position of vacuum advance mechanism relative to
engine. Lift distributor straight up to complete
removal. Remove distributor flange to cylinder
block gasket.
N O T E : Always set distributor in upright posi­
tion so oil from distributor shaft will not run out
onto the breaker plate and points.
DISTRIBUTOR INSTALLATION
IF EN G IN E HAS N O T BEEN C R A N K E D
1. If distributor is new, No. 1 firing position
can be determined by the mark made on the old
distributor housing prior to removal. Also, locate
mark made on housing after gears were disengaged.
2. Place new distributor to cylinder block gas­
ket on block, and lubricate distributor drive gear
with engine oil.
3. Turn rotor so segment points to the mark
made after disengaging gears. As distributor is
SCREWDRIVER
Figure 6— Setting Point Opening
CHEVROLET SERIES 70-80 HEAVY DUTY TRUCK SH O P M A N U A L
ENGINE ELECTRICAL 6Y-15