Chevrolet Light Duty Truck 1973 Service Manual page 338

6-2 ENGINE
enough force to remove the boot. Do not pull on plug
wire.
TEST COMPRESSION (FIG. 1)
The compression check is important because an engine
with low or uneven compression cannot be tuned
successfully. It is essential that improper compression be
corrected before proceeding with the engine tune up.
1. Remove air cleaner and block throttle and choke in
wide open position.
2. Hook up starter remote control cable and insert
compression gauge firmly in spark plug port.
CAUTION:
Whenever the engine i s cranked
remotely at the s t a r t e r , with a special jumper
cable or other means, the distributor primary
lead must be disconnected from the negative
post on the c o i l .
3. Crank engine through at least four compression
strokes to obtain highest possible reading.
4. Check and record compression of each cylinder.
5. If one or more cylinders read low or uneven, inject
about a tablespoon of engine oil on top of pistons
in low reading cylinders (through spark plug port).
Crank engine several times and recheck
compression.
• If compression comes up but does not necessarily
reach normal, rings are worn.
• If compression does not improve, valves are
burned, sticking or not seating properly.
• If two adjacent cylinders indicate low compres­
sion and injecting oil does not increase
compression, the cause may be a head gasket
leak between the cylinders. Engine coolant and/
or oil in cylinders could result from this defect.
NOTE: If a weak cylinder cannot be located
Fig. 1—Checking Compression
with the compression check, see "Cylinder
Balance Test" under "Additional Checks and
Adjustments" in this section.
SERVICE AND INSTALL SPARK
PLUGS (FIG. 2 )
1. Inspect each plug individually for badly worn
electrodes, glazed, broken or blistered porcelains
and replace plugs where necessary.
2. Clean serviceable spark plugs thoroughly, using an
abrasive-type cleaner such as sand blast. File the
center electrode flat.
3. Inspect each spark plug for make and heat range.
All plugs must be of the same make and number.
4. Adjust spark plug gaps to specifications using a
round feeler gauge.
Never bend the center electrode t o
CAUTIO N:
adjust gap. Always adjust by bending ground
or side e l e c t r o d e .
5. If available, test plugs with a spark plug tester.
. Inspect spark plug hole threads and clean before
6
installing plugs. Corrosion deposits can be removed
with a 14 m m .x 1.25 SAE spark plug tap (available
through local jobbers) or by using a small wire
brush in an electric drill. (Use grease on tap to
catch chips.)
Use extreme care when using tap t o
CAUTIO N:
prevent cross threading. Also crank engine
several times t o blow out any material
dislodged during cleaning operation.
7. Install spark plugs and torque to specifications.
NOTE: The following are some of the
greatest causes of unsatisfactory spark plug
performance.
• Installation of plugs with insufficient torque to
fully seat.
• Installation of the plugs using excessive torque
which changes gap settings.
• Installation of plugs on dirty seat.
• Installation of plugs to corroded spark plug hole
threads.
. Connect spark plug wiring.
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SERVICE IG N ITIO N SYSTEM
1. Remove distributor cap, clean cap and inspect for
cracks, carbon tracks and burned or corroded
terminals. Replace cap where necessary (fig. 3).
2. Clean rotor and inspect for damage or deteriora­
tion. Replace rotor where necessary.
3. Replace brittle, oil soaked or damaged spark plug
wires. Install all wires to proper spark plug. Proper
positioning of spark plug wires in supports is
important to prevent cross-firing.
LIGHT DUTY TRUCK SERVICE MANUAL