454 CID (7.4L) / 502 CID (8.2L)
SERVICE MANUAL NUMBER 23
Page 3A-23
90-861326--1 MARCH 1999
3. Any other protuberance in the combustion chamber, such as an overhanging piece of
gasket, an improperly seated valve or any other inadequately cooled section of material
which can serve as a source for pre-ignition problems.
NOTE: Engine failures, which result from the foregoing conditions, are beyond the control
of Mercury Marine; therefore, no warranty will apply to failures which occur under these con-
ditions.
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 damage to the spark plugs, pistons, valves and, in severe cases,
results in pre-ignition.
Use of low octane gasoline is one of the most common causes of detonation. Even with high
octane gasoline, detonation could occur if engine maintenance is neglected.
OTHER CAUSES OF DETONATION
IMPORTANT: Use of improper fuels will cause engine 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 compression ratio).
5. Inadequate cooling of engine by deterioration of cooling system.
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