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ENGINE 6-8
Check and Adjust Ignition Timing (Fig. 13)
1. Disconnect the distributor spark advance hose and
plug the vacuum source opening.
2. Start engine and run at idle speed (see tune up chart).
3. Aim timing light at timing tab.
NOTE: - The markings on the tabs are in 2°
increments (the greatest number of markings on
the "A" side of the "O"). t h e "O" markings is
TDC of #1 cylinder and all BTDC settings fall on
the "A" (advance) side of "O".
4. Adjust the timing by loosening the distributor clamp
and , rotating the distributor body as required, then
tighten the clamp.
5. Stop engine and remove timing light and reconnect
the spark advance hose.
Adjust Idle Speed and Mixture (Fig. 14) (Except when
equipped with Air Injection Reactor System)
1. As a preliminary adjustment, turn idle mixture
screws lightly to seat and back out 2 turns.
CAUTION:
Do not turn idle mixture screw
tightly against seat or damage may result.
2. With engine running at operating temperature (choke
wide open) adjust idle speed screw to bring idle
speed to specified rpm (automatic transmission in
drive, manual transmission in neutral).
3. Adjust idle mixture screw to obtain highest steady
idle speed (1/4 turn out from lean roll).
4. Repeat Steps 2 and 3 as needed for final adjustment.
5. Shut down the engine, remove gauges and install air
cleaner.
Adjust Idle Speed and Mixture (Fig. 14) (With Air
Injection Reactor System)
The recommended adjustment procedure for Air Injec-
tion Reactor System equipped engines is as follows:
1. As a preliminary adjustment, turn idle mixture
screws lightly to seat and than back out 3 turns.
CAUTION:
Do not turn idle mixture screw
tightly against seat or damage may result.
2. With engine running at operating temperature, choke
wide open, and parking brake applied, adjust idle
specified idle speed (automatic
"drive"-manual transmission in
to
in
screw
transmission
"neutral").
;3. Adjust idle mixture screw (turn in) to "lean roll"
position; then turn screw out 1/4 turn (1/4 turn rich
from "lean roll"). The definition of "lean roll" point
is a 20 to 30 rpm drop in engine speed, obtained by
leaning the idle mixture.
4. Repeat Steps 2 and 3 as needed for final adjustments.
ADDITIONAL CHECKS AND ADJUSTMENTS
Testing Crankcase Ventilation Valve (Fig. 15)
0
1. Connect tachometer and vacuum gauge as for idle
speed and mixture adjustment.
2. Set parking brake, start engine and adjust idle speed
and mixture.
3. Disconnect ventilation hose at valve, block opening of
valve and read engine rpm change.
4. A change of less than 50 rpm indicates a plugged
ventilation valve - replace the valve.
Cylinder Balance Test (Fig. 16)
It is often difficult to locate a weak cylinder. A com-
pression test, for example, will not locate a leaky intake
manifold, a valve not opening properly due to a worn
camshaft, or a defective spark plug.
With the cylinder balance test, the power output of one
cylinder may be checked against another, using a set of
grounding leads. When the power output of each cylinder
is not equal, the engine will lose power and run roughly.
Perform a cylinder balance test as follows:
1. Connect the tachometer and vacuum gauge.
2. Start engine and run at 1500 rpm.
3. Ground large clip of grounding leads and connect in-
dividual leads to all spark plugs except the pair being
tested.
Divide the firing order in half and arrange one half
over the other. The cylinders to be tested together ap-
pear one over the other.
L4 Firing Order
V8 Firing Order
1-8-4-3-6-5-7-2
1-6, 8-5, 4-7, 3-2
1-3-4-2 = 1-3
4-2
L6 Firing Order
1-5-3-6-2-4 =
= 1-4. 3-2
1-5-3
6-2-4
1-6, 5-2, 3-4
1-8-4-3
6-5-7-2
4. Operate engine on each pair of cylinders in turn and
note engine rpm and manifold vacuum for each pair.
A variation of more than 1 inch of vacuum or 40 rpm
between pairs of cylinders being tested indicates that
the cylinders are off balance.
Battery
The battery should be checked with special testing
equipment and to the equipment manufacturers specifica-
tions. See Section 6Y for complete information on battery
tests.
Ignition
The following additional ignition checks may be made
with any of several pieces of equipment available for un-
covering the source of engine difficulties. The specific
operating instructions of the equipment manufacturer
should be followed:
Cranking voltage
Ignition switch
Distributor resistance
Secondary resistance
Ignition output and secondary leakage
Cranking Voltage (Fig. 17)
1. Disconnect coil primary lead at the coil negative
terminal to prevent engine from firing during
cranking.
2. Connect voltmeter between primary terminal of coi|
(resistance wire side) and ground.
3. Operate starting motor.
CHEVROLET CHASSIS SERVICE MANUAL
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