GMC 4000 Series Service Manual page 413

Sec. 6Y
Page 402
IGNITION SYSTEM
Figure 1 — D iseng a g in g Ignition Switch Connector
3. Remove the metal ignition switch retaining
nut from the passenger side of the dash, or from
top of transm ission control island.
4. Pull ignition switch out from
transm ission control island and separate wiring
connectors by inserting thin blade of small screw ­
driver under each tang of wiring connector as
shown in figure 1. Pull connector from switch.
INSTALLATION
1. Engage lock tangs of wiring connector to
switch. Make sure lock tangs are fully engaged by
trying to separate.
2. Place the switch with wiring into position
from underside of dash or transmission control
island and install switch retaining nut firmly.
3. Insert switch lock cylinder in switch hous­
ing and rotate clockwise to secure in "LOCK"
position.
4. Connect battery negative cable to post and
check operation of switch.
Figure 2 — Tim ing M a rk s (In-Line Engine) (Typical)
G M C S E R V I C E M A N U A L
dash or
under
Timing the ignition system comprises:
(1) Initial Timing — Setting distributor to permit
(2) Manual Advance Adjustment — Retarding or
nition timing, make sure carburetor is properly
adjusted. Disconnect vacuum line from distributor
vacuum advance unit and plug the open end of line.
ommended settings for average nation-wide regu­
lar gasoline. Timing must be retarded as required
when lower octane gasoline is used.
IN-LINE ENGINES
(WITH ENGINE RUNNING)
engine, timing is set at 4° BUDC on No. 1 cylinder.
Timing tab is located at front of engine above the
crankshaft pulley at left side (fig. 2).
are in 2° increments. The "O" mark is upper-
dead-center (UDC) and the 4 mark is the second
mark toward "A."
plug and with engine idling (500 rpm max.) loosen
mounting clamp cap screw at base of distributor
(fig. 4) and rotate distributor as necessary to syn­
chronize flashes with timing mark when mark is
Figure 3 — Timing Marks (V6 Engine) (Typical)
IGNITION TIMING
opening of points at correct firing intervals
- and -
advancing the point opening to compensate for
various grades of fuel which may be used. These
timing factors require checking and adjusting
at regular intervals, or whenever performance
of engine necessitates such action.
IMPORTANT: Before attempting to adjust ig­
NOTE: The following timing settings are rec­
On vehicles equipped with the L-250 or L-292
On all In-line engines, markings on timing tab
With timing light connected to No. 1 spark
P O IN T E R