Mercury 305 CID, Service Manual, Page: 213 / 967

ENGINES - 3A-21 90-823225--1 1096
Detonation
Detonation, commonly called “fuel knock,” “spark
knock” or “carbon knock,” is abnormal combustion of
the fuel which causes the fuel to explode violently.
The explosion, in turn, causes overheating or dam-
age to the spark plugs, pistons, valves and, in severe
cases, results in pre-ignition.
Use of low octane gasoline is one of the most com-
mon causes of detonation. Even with high octane
gasoline, detonation could occur if engine mainte-
nance is neglected.
OTHER CAUSES OF DETONATION
IMPORTANT: Use of improper fuels will cause en-
gine damage and poor performance.
1. Over-advanced ignition timing.
2. Lean fuel mixture at or near full throttle (could be
caused by carburetor or leaking intake manifold).
3. Cross-firing spark plugs.
4. Excess accumulation of deposits on piston and/
or combustion chamber (results in higher com-
pression ratio).
5. Inadequate cooling of engine by deterioration of
cooling system.
NOTE: Engine failures, which result from the forego-
ing conditions, are beyond the control of MerCruiser;
therefore, no warranty will apply to failures which oc-
cur under these conditions.
72425
Detonation Damage
72315
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