GMC 1976 ZEO 6083 Maintenance Manual page 279

6A- 1 4 ENGINE
SPARK KNOCK, PING, OR DETONATION
NOTE: A sharp metallic knock due to instantaneous abnormal combustion .
GENERAL
Low octane fuel .
Too high compression .
Timing advanced too far.
Heavy carbon deposits .
Manifold heat control valve faulty .
Faulty distributor advance mechanism.
Breaker point dwell (or gap) too low.
ENGINE CONTINUES TO RUN AFTER IGNITION IS TURNED OFF
(DIESELING)
NOTE : When the engine won't stop as the ignition is turned off, the cause is often
due to red hot carbon particles resting on heavy carbon deposit in a very hot
engine .
GENERAL
Improper idle speed (too high) . (1)
High engine temperature .
Poor grade fuel (octane too low) .
Improper timing and dwell .
Quick shut-down of hot engine .
MECHANICAL
Improper valve timing .
NOTE : Hot spot in combustion chamber ignites fuel before spark occurs . May
not be noticed unless severe .
GENERAL
Overheated engine .
Carbon deposits .
Spark plugs not tight.
Spark plugs with wrong heat range.
Timing and dwell improperly set.
PRE-IGNITION
COOLING
Overheated engine . (See "Engine Overheats"
in this section.)
Hot weather.
High altitude .
IGNITION
Advanced timing .
Improper heat range or improperly installed
spark plugs.
Electrical feed through ignition system
(faulty switch).
FUEL
Carburetor too lean .
Throttle plates misaligned .
MECHANICAL
Leak at valve due to clearance, valve
sticking, weak or broken spring .
Valve timing .