Chevrolet 10 Series 1967, Service Manual

Page: 288 / 605

ENGINE 6-8
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
speed screw to specified idle speed (automatic
transmission in "d riv e” - manual transmission in
"neutral” ).
ADDITIONAL CHECKS
Testing Crankcase Ventilation Valve (Fig. 14)
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. 15)
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
individual 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.
L
6
Firing Order
1 -5 -3 -6 -2 -4 = 1-5-3 = 1-6, 5-2, 3-4
6-2-4
V
8
Firing Order
1 -8 -4 -3 -6 -5 -7 -2 = 1-8-4-3 = 1-6, 8-5, 4-7, 3-2
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.
F ig . 14—Crankcase V entilatio n V a lve
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 adjustment.
AND ADJUSTMENTS
Battery
Two battery checks should be made to determine the
condition of the battery in a minimum amount of time.
1. Hydrometer Test
2. Light Load Test
NOTE: If a battery failure is encountered, the
cause may be outside the battery itself. Do not
be satisfied merely to recharge or replace it.
Find the cause of failure and prevent reoccur­
rence.
See Section
6
Y for a description of above tests.
Ignition
The following additional ignition checks may be made
with any of several pieces of equipment available for
uncovering 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
C
ranking Voltage (Fig. 16)
1. Disconnect coil primary lead at the coil negative
terminal to prevent engine from firing during
cranking.
2. Connect voltmeter between primary terminal of coil
(resistance wire side) and ground.
3. Operate starting motor.
a. If voltage is 9 volts or more and cranking speed
is satisfactory, the battery, starter, cables,
CHEVROLET TRUCK SERVICE M ANUAL