GMC 4000 Series Service Manual page 414

RO TO R
DISTRIBUTOR
C A P SC R E W
M O U N T IN G C LAM P
Figure 4 — Distributor Installed (with Cap
Removed (In-Line Engine) (Typical)
aligned with pointer. After completing adjustment,
tighten cap screw and connect vacuum line.
V-6 ENGINES
(WITH ENGINE RUNNING) (Fig. 3)
NOTE: Make sure distributor is in good con­
dition and dwell angle is properly adjusted before
checking ignition timing. If dwell angle requires
adjustment, adjust as directed later under "Dis­
tributor."
1. Figure 3 shows location of timing marks
on the crankshaft pulley hub. Connect one lead of
a timing light to No. 1 sparkplug terminal (front
plug on left side) and connect the other lead in
accordance with instructions furnished with the
instrument.
2. Start engine and run at idle (550rpmmax.).
3. On 305-C or 305-E engines, set timing to
V/2 before upper-dead-center and on 351-C or
351-M engines, set timing to 10° BUDC. If timing
mark does not align with pointer, loosen distrib­
utor mounting clamp cap screw (fig. 5) and rotate
distributor body as required to synchronize timing
light flashes with timing mark on engine crank­
shaft pulley.
4. Tighten distributor mounting clamp cap
screw and connect vacuum advance line after ad­
justment is completed.
IN-LINE AND V-6 ENGINES
(WITH ENGINE NOT RUNNING)
1. Locate No. 1 cylinder spark plug wire on
distributor cap; mark distributor body adjacent to
No. 1 wire socket in cap, then remove cap.
2. Intermittently operate starter until proper
G M C S E R V I C E M A N U A L
V A C U U M A D V A N C E
UN|T
J w J H B H ~ 3 0 9 5 .
timing mark on crankshaft pulley is aligned with
timing tab or pointer (fig. 2 or 3).
pointer, rotor segment should point toward mark
made on distributor body in Step 1 previously. In­
stead, rotor segment may point 180 degrees away
from mark; in this case engine must be rotated one
complete revolution and timing mark realigned.
on pointer and with rotor segment pointing to No. 1
spark plug wire, points should just begin to open.
Loosen distributor mounting clamp cap screw and
turn distributor housing clockwise until points
close. Remove high tension wire from center socket
in distributor cap. Turn on ignition switch and hold
end of high tension wire (still connected to coil)
%-inch from a ground; then turn distributor housing
counterclockwise until a spark jumps the gap be­
tween high tension wire and ground. When spark
occurs, points are open. Hold distributor in this
position and tighten mounting clamp cap screw.
Turn ignition switch off and install distributor cap.
Install high tension wire in cap.
MANUAL ADVANCE ADJUSTMENT
up, drive vehicle using grade of fuel expected to be
used in service. Engine should not ping or knock
excessively under load and full throttle.
mounting clamp cap screw (fig. 4 or 5) and turn
distributor housing clockwise to retard spark until
knock is eliminated.
the best possible engine performance with thepar-
ticular grade of gasoline being used.
DISTRIBUTOR
CAP
SC REW
DISTRIBUTOR
H O U S IN G
— V I m r l CAP SC REW
M O U N T IN G
CLAM P
Figure 5 — Distributor and Ignition Coil
Installed (V6 Engine) (Typical)
With timing mark aligned with timing tab, or
3. With timing mark aligned with timing tab
1. After engine has been thoroughly warmed
2. If knock is evident, loosen the distributor
NOTE: Manual advance should be set to obtain
Sec. 6Y
Page 403
IGNITION SYSTEM
C O N D E N SE R
COIL
1 -4.
t
T-441 9