Chevrolet 10 Series 1970 Service Manual page 332

ENGINE 6-10
with manual transm ission in neutral; automatic
tran sm ission in d riv e .
Adjust m ixture screw s equally in to obtain a 20
rpm drop, then back screw s out 1/4 turn.
ADDITIONAL CHECKS
Testing Crankcase Ventilation Valve (Fig. 18)
1. Connect tach om eter and vacuum gauge as fo r idle
speed and m ixture adjustment.
2. Set parking brake, start engine and adjust idle speed
and m ixture.
3. R em ove vent va lv e (P C V ) fro m ro ck e r co ve r grom -
m et (with hose attached, block opening o f valve and
read engine rpm change.
4. A d ecrease o f le s s than 50 rpm indicates a plugged
ventilation va lve - rep lace the va lve.
Cylinder Balance Test (Fig. 19)
It is often d ifficu lt to locate a weak cylin der. A com ­
pression test, fo r exam ple, w ill not locate a leaky intake
m anifold, a va lve not opening p ro p e rly due to a worn
cam shaft, or a d e fe ctive spark plug.
With the cylin d er .balance test, the pow er output o f one
cylin der may be checked against another, using a set o f
grounding leads. When the pow er output o f each cylin der
is not equal, the engine w ill lo se pow er and run roughly.
P e r fo r m a cylin d er balance test as follow s:
1. Connect the tach om eter and vacuum gauge.
2. Start engine and run at 1500 rpm.
3. Ground la rg e c lip o f grounding leads and connect
individual leads to a ll spark plugs except the pair
being tested.
D ivide the firin g o rd e r in half and arrange one half
o v e r the other. The cylin d ers to be tested together ap­
p ea r one o v e r the other.
L8 F irin g O rd er
1 -5 -3 -6 -2 -4 = 1-5-3 = 1-6, 5-2, 3-4
6-2-4
V8 F irin g O rd er
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. O perate engine on each p a ir o f cylin d ers in turn and
note engine rpm and m anifold vacuum fo r each pair.
A variation o f m ore than 1 inch o f vacuum o r 40 rpm
between p a irs of cylin d ers being tested indicates that
the cylin d ers a re o ff balance.
F ig . 18—C rankcase V e n tila tio n V a lv e
Adjust, if n ecessary, idle speed screw to obtain
500 rpm with manual tran sm ission in neutral;
automatic transm ission in d rive,
AND ADJUSTMENTS
Battery
The battery should be checked with special testing
equipment and to the equipment m anufacturers s p e c ifi­
cations. See Section 6Y fo r
battery tests.
Ignition
The follow in g additional ignition checks may be made
with any o f s e v e ra l p ieces o f equipment available fo r
uncovering the source o f engine d ifficu lties. The specific
operating instructions o f
should be follow ed.
• Cranking voltage
• Ignition switch
• D istributor resistan ce
• Secondary resistance
• Ignition output and secondary leakage
Cranking V oltage (Fig. 20 )
1. Disconnect co il p rim a ry lead at the coil negative
term in al to prevent
cranking.
2. Connect voltm eter between p rim a ry term in al o f coil
(resistan ce w ire side) and ground.
3. O perate starting m otor.
a. If voltage is 9 volts or m ore and cranking speed
is sa tisfactory, the
sta rter switch and ignition circu it to c o il (by­
passing resistan ce w ir e ) a re in good condition.
b. If below 9 volts, check circu it until d ifficu lty is
located.
M eter reading below
tery; d efective cables,
starter; d efective ignition circu it to coil.
Cranking speed below n orm a l—E xcessive r e ­
sistance in cables or starting m otor; ex cessive
com plete inform ation on
the equipment manufacturer
engine from firin g during
battery, sta rter, cables,
sp ecifica tion -W eak bat­
connections, switch or
CHEVROLET TRUCK SERVICE MANUAL