Pontiac FIREBIRD 1972 Service Manual page 738

6C-2 1972 PONTIAC SERVICE MANUAL
Example 1
CYLINDER PRESSURE
1
....................................................................
146
2
....................................................................
148
3
....................................................................
145
4
....................................................................
154
5
....................................................................
112
6
....................................................................
147
7
....................................................................
150
8
....................................................................
151
80% of 154 (highest) is 123. Thus cylinder No. 5 is less
than 80% of No. 4. This condition, accompanied by low
speed missing, indicates a burned valve or broken piston
ring.
Example 2
CYLINDER PRESSURE
1
......................................................................
95
2
....................................................................
101
3
....................................................................
100
4
......................................................................
97
5
...................................................................
106
6
....................................................................
103
7
......................................................................
97
8
......................................................................
99
80% of 106 is 85. While all cylinders are well above 85,
they are all excessively low. This may indicate poor valves
in all cylinders or, if accompanied by oil consumption,
worn rings or low cranking speed. If compression is sub-
normal, tune-up will probably not be satisfactory. (See
specifications at end of section 6 for correct compression
pressures.)
NOTE: The compression check is important be-
cause an engine with low or uneven compres-
sion cannot
be
tuned successfully to give peak
perfonname. Therefore, it is essential that im-
proper compressin be corrected before pro-
ceeding with an engine tune-up.
CLEAN, TEST A N D INSTALL SPARK
PLUGS
1. Inspect each plug individually for badly worn elec-
trodes, glazed, broken or blistered porcelains and re-
place plugs where necessary. For optimum engine
performance and economy, it is recommended that
spark plugs be replaced every 12,000 miles. Refer to
the spark plug diagnosis information presented in
Engine Electrical for an analysis of plug conditions.
2. Clean serviceable spark plugs thoroughly, using an
abrasive-type cleaner. 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 or heat
range. (See section 6E for correct spark plug usage.)
4. Adjust spark plug gaps to .035" using a round feeler
gauge.
CAUTION: Never bend the center electrode to
adjust gap. Always adjust by bending ground
(or side) electrode.
5. If available, test plugs with a spark plug tester.
6. Inspect spark plug hole threads and clean before in-
stalling plugs. Corrosion deposits can be removed
with a 14 mm.
x
1.25 SAE spark plug tap (available
through local jobbers) or by using a small wire brush
in an electric drill. Use plenty of grease on the tap to
catch any chips.
CAUTION: Use extreme care when using tap to
prevent cross threading. Also crank engine sev-
eral times to blow out any material dislodged
dunng cleaning operation.
7. Install spark plugs in engine and tighten to 15 lb. ft.
(6-cyl. and V-8).
Improper installation is one of the greatest single
causes of unsatisfactory spark plug performance, and
is the result of one or more of the following practices:
Installation of plugs with insufficient torque to
fully seat the plug.
Installation of the plugs using excessive torque
which changes gap settings.
Installation of plugs on dirty seal.
Installation of plugs to corroded spark plug hole
threads.
Failure to install plugs properly will cause them to operate
at excessively high temperatures and result in reduced
operating life under mild operation or complete destruc-
tion under severe operation where the intense heat cannot
be dissipated rapidly enough.
Always remove corrosion deposits in hole threads before
installing plugs. When corrosion is present in threads,
normal torque is not sufficient to compress the plug gasket
and early failure from overheating will result.