Mariner 100, Service Manual, Page: 17 / 391

1-12 90-13645--2 495 GENERAL INFORMATION AND SPECIFICATIONS
Detonation: Causes and Prevention
Detonation in a 2-cycle engine somewhat resembles
the “pinging” heard in an automobile engine. It can be
otherwise described as a tin-like “rattling” or “plinking”
sound.
Detonation generally is thought of as spontaneous ig-
nition, but it is best described as a noisy explosion in
an unburned portion of the fuel/air charge after the
spark plug has fired. Detonation creates severe, un-
timely, shock waves in the engine, and these shock
waves often find or create a weakness: The dome of
a piston, piston rings or piston ring lands, piston pin
and roller bearings.
While there are many causes for detonation in a
2-cycle engine, emphasis is placed on those causes
which are most common in marine 2-cycle applica-
tion. A few, which are not commonly understood, are:
1. Over-advanced ignition timing.
2. Use of low octane gasoline.
3. Propeller pitch too high (engine RPM below rec-
ommended maximum range).
4. Lean fuel mixture at or near wide-open-throttle.
5. Spark plugs (heat range too hot – incorrect reach
– cross-firing).
6. Inadequate engine cooling (deteriorated cooling
system).
7. Combustion chamber/piston deposits (result in
higher compression ratio).
Detonation usually can be prevented, provided that
1) the engine is correctly set up and 2) diligent mainte-
nance is applied to combat the detonation causes,
listed, preceding.
51115
Damaged Piston Resulting from Detonation
Compression Check
1. Remove spark plugs.
2. Install compression gauge in spark plug hole.
3. Hold throttle plates at W.O.T.
4. Crank engine thru at least 4 compression strokes
to obtain highest possible reading.
5. Check and record compression of each cylinder.
Variation of more than 15 psi (103.5 kPa) between
cylinders indicates that lower compression cylin-
der is in some way defective, such as worn or
sticking piston rings and/or scored piston and cyl-
inder.
6. Compression check is important because an en-
gine with low or uneven compression cannot be
tuned successfully to give peak performance. It is
essential, therefore, that improper compression
be corrected before proceeding with an engine
tune-up.
7. Cylinder scoring: If powerhead shows any indica-
tion of overheating, such as discolored or
scorched paint, visually inspect cylinders for scor-
ing or other damage as outlined in Section 4
“Powerhead.”