GMC 4000 Series Service Manual page 417

Sec. 6Y
Page 406
IGNITION SYSTEM
TO SP A R K PORT
A B O V E CAR BU RETO R
THROTTLE PLATES
TO SP A R K PORT
B E L O W CAR BU RETO R
THROTTLE PLATES
Figure 9 — Position of Vacuum Hoses (with C.C.S.) (Typical)
Centrifugal 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 var­
ious speed conditions.
V A CU U M 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.
On vehicles equipped with the controlled com­
bustion system , a double-acting diaphragm is used
in the distributor vacuum control unit. The vacuum
retard side of the diaphragm is connected to mani­
fold vacuum through ports on carburetor. At idle
speed, with carburetor throttle valves closed, vac­
uum on this diaphragm will retard the spark 10
degrees. The opposite side of the diaphragm is
connected to a port above the throttle valves (fig.
9). When carburetor throttle valves are open, vac­
uum on this side of the diaphragm immediately
advances the spark to the initial setting. When
throttle valves are open, the double-acting vacuum
advance unit functions exactly the same as the
single diaphragm vacuum advance unit.
G M C S E R V I C E M A N U A L
distributor cap, mark this position on cap and
mark distributor housing adjacent to No. 1 wire in
cap.
then remove cap and place clear of work area.
terminal on ignition coil.
shaft pulley comes to rest with the correct timing
mark aligned with pointer or timing tab (fig. 2 or 3).
tion for ignition timing point on each engine.
segment should point toward mark made on dis­
tributor body in Step 1 previously; instead, rotor
T-464 4.^
segment may point 180 degrees away from mark;
in this case, rotate engine one complete revolution
and realign timing mark with pointer.
reinstalled if engine remains in this position.
then remove cap screw and mounting clamp (fig.
4 or 5). Lift distributor straight up until spiral
gear disengages, 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 rel­
ative to engine. Lift distributor straight up to com­
plete removal. Remove distributor flange to cyl­
inder block gasket.
tion so oil from distributor shaft will not run out
onto breaker plate and points.
IF ENGINE HAS NOT BEEN CRANKED
(IN-LINE ENGINE MODELS)
a clockwise direction past the mark previously
placed on the distributor housing to locate the rotor.
ket on block, and lubricate distributor drive gear
with engine oil.
the block with the distributor housing in a normal
"installed" position.
slightly to start gear into mesh with camshaft gear,
but rotor should line up with the mark on distrib­
utor housing when distributor is in place.
and connect vacuum line. Connect primary wire to
coil terminal and install cap. Install spark plug
and high tension wires if removed.
DISTRIBUTOR REMOVAL
1. Locate No. 1 cylinder spark plug wire on
2. Release distributor cap hold-down screws;
3. Disconnect distributor primary wire from
4. Intermittently operate starter until crank­
NOTE: Refer to "Ignition Timing" in this s e c ­
With pointer and timing mark aligned, rotor
NOTE: Distributor can be removed and readily
5. Disconnect vacuum line(s) from distributor,
NOTE: Always set distributor in upright posi­
DISTRIBUTOR INSTALLATION
1. Turn the rotor approximately 1/8 turn in
2. Place new distributor to cylinder block gas­
3. Push the distributor down into position in
NOTE: It may be necessary to move rotor
4. Tighten the distributor clamp bolt snugly
5. Time ignition as previously described.