GMC 1976 ZEO 6083 Maintenance Manual page 10

23 CARBURETOR CHOKE-Check choke mech-
anism for free operation. Any binding condition
which may have developed due to petroleum gum
formation on the choke shaft or from damage should
be corrected .
24 TIMING, DWELL, CARBURETOR IDLE, DISTRIBU-
TOR AND COIL-Adjust ignition timing, dwell and
carburetor idle speed accurately (following the
specifications shown on the label attached to engine
rocker cover) at the first 6 months or 6,000 miles of
operation then at 12 month or 12,000 miles. Then at
12 month or 12,000 mile intervals. Adjustment must
be made with test equipment known to be accurate.
Replace distributor points every 12 months or
12,000 miles and replace cam lubricator every 24
months or 24,000 miles. In addition, carefully in
spect the interior and exterior of the distributor cap,
distributor rotor and coil for cracks, carbon tracking,
and terminal corrosion. Clean or replace as necessary
at 24-month/24,000 mile intervals to prevent misfir-
ing and/or deterioration.
Proper functioning of the carburetor is particu-
larly essential to control of emissions. Correct mix-
tures for emission compliance and idle quality have
been preset by GMC Truck. Plastic idle mixture li-
miters have been installed on the idle mixture screws
to discourage unauthorized adjustment . These idle
limiters are not to be removed unless some major
carburetor repair or replacement which affects the
idle screw adjustment has been necessary.
At 12 months or 12,000 mile intervals or in case
of major carburetor overhaul, or when poor idle
quality exists, the idle mixture should be adjusted by
use of a CO meter when an accurate meter is availa-
ble, or the alternate mechanical method should be
used to adjust idle mixture (See SECTION 6M).
25 CARBURETOR AND INTAKE MANIFOLD
M O U N T I N G - T o r q u e c a r b u r e t o r a n d i n t a k e m a n i f o l d a t t a c h -
ing bolts and/or nuts at first 6 months or 6,000 miles
of vehicle operation.
26 SPARK PLUGS-Replace at 6,000 mile intervals
when operating with leaded fuels, or at 12,000-mile
intervals when using unleaded fuels. Use of leaded
fuels results in lead deposits on spark plugs and can
cause misfiring at mileages less than 12,000 miles.
Where misfiring occurs prior to 6,000 miles, spark
plugs in good condition can often be cleaned, tested,
and reinstalled in an engine with acceptable results .
27 THERMAL VACUUM SWITCH AND HOSES-
Check for proper operation . A malfunctioning
switch must be replaced . Check hoses for proper
connection, cracking, abrasion or deterioration and
replace as necessary .
GENERAL INFORMATION
0- 9
28 CARBURETOR FUEL FILTER-Replace filter at
12-month/12,000-mile intervals or more frequently
if clogged.
29 POSITIVE CRANKCASE VENTILATION SYSTEM
(PCV)-Check system for satisfactory operation at 12-
month or 12,000-mile intervals using a tester, and
clean filter . Replace the PCV valve at 24-month or
24,000-mile intervals and blow out PCV valve hose
with compressed air. The PCV valve should be re-
placed at 12-month or 12,000-mile intervals when
the vehicle is used in operations involving heavy
dust, extensive idling, trailer pulling, and short trip
use at freezing temperatures where engine does not
become thoroughly warmed-up.
30 AIR CLEANER ELEMENT-Replace the engine air
cleaner element under normal operating conditions
every 12,000 miles. Operation of vehicle in dusty
areas will necessitate more frequent element replace-
ment .
CAUTION : Do not operate the engine with-
out the air cleaner unless temporary re-
moval is necessary during repair or
maintenance of the vehicle . When the air
cleaner is removed backfiring can cause fire
in the engine compartment.
31 SPARK PLUG AND IGNITION COIL WIRES-In-
spect spark plug and ignition coil wires for evidence
of checking or cracking of exterior insulation and
tight fit in the distributor cap and at the spark plugs.
Exterior of wires should be cleaned, any evidence of
corrosion on ends removed and wire replaced if
deteriorated .
32 ENGINE COMPRESSION-Test engine cranking
compression. If a problem exists, have correction
made . Minimum compression recorded in any one
cylinder should not be less than 70% of the highest
cylinder . For example, if the highest pressure in any
one cylinder is 1501bs ., the lowest allowable pressure
for any other cylinder would be 105 lbs. (150 X 70%
= 105) .
33 EVAPORATION CONTROL SYSTEM (ECS)-Check
all fuel and vapor lines and hoses for proper connec-
tions and correct routing as well as condition .
Remove canister(s) and check for cracks or damage .
Replace damaged or deteriorated parts as necessary.
Replace filter in lower section of canister .
If vehicle is equipped with two canisters, filter is
located in the lower canister only .
34 FUEL CAP, FUEL LINES AND FUEL TANKS-In-
spect the fuel tank cap and lines for damage which
could cause leakage. Inspect fuel cap for correct seal
ing ability and indications of physical damage . Re-
place any damaged or malfunctioning parts.