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What Causes Detonation in a 2-Stroke Outboard?

Detonation in a 2-cycle outboard engine resembles the “pinging” heard in an automobile engine. It can be otherwise described as a tin-like “rattling” or

“plinking” sound but most of the time you can't hear it to due to the engine, wind, and water noises.

Detonation is an explosion of an unburned portion of the fuel/air charge after the spark plug has fired. Detonation creates severe shock waves in the

engine and these shock waves often find or create a weakness that can quickly destroy the top of a piston, cylinder head/gasket, piston rings or piston ring lands, piston pin, and roller bearings.

A few of the most common causes of detonation in a marine 2-cycle outboard or jet boat powerhead are as follows:

1. Over-advanced ignition timing. This could happen simply by the top throttle arm or the linkage becoming loose and moving the timing adjustor. Or the timing advance box malfunctioning, all unsuspectedly.

2. Use of low-octane gasoline. The lower the octane the faster the fuel burns, and if the fuel burns before the piston has a chance to reach top dead center, it pounds the top of one or more pistons.

3. Propeller pitch too high (engine RPM below recommended maximum range). AKA lugging the motor.

4. Lean fuel mixture at or near wide-open throttle.

5. Wrong or bad spark plugs (heat range too hot, incorrect reach, gap, or cross-firing).

6. Inadequate engine cooling (deteriorated cooling system, or bad water pump).

7. Combustion chamber/piston deposits (result in higher compression ratio).

There are other reasons for blown outboard motors, stuck or melted pistons, and broken rods; feel free to email STVracer photos and we can help you identify what went wrong.

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