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Spark plugs are one of the most misunderstood components of an engine.
Numerous questions have surfaced over the years, leaving many people
confused. Spark plug reading is a science all it?s own, but there are
some rules of thumb.
Spark plugs are the "window" into your engine (your only
eyewitness to the combustion chamber), and can be used as a valuable
diagnostic tool. Like a patient's thermometer, the spark plug displays
symptoms and conditions of the engine's performance. The experienced tuner
can analyze these symptoms to track down the root cause of many problems,
or to determine air/fuel ratios.
SPARK PLUG BASICS:
The spark plug has two primary functions: |
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- To ignite the air/fuel mixture
- To remove heat from the combustion
chamber
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Spark plugs transmit electrical energy that turns fuel into working
energy. A sufficient amount of voltage must be supplied by the ignition
system to cause it to spark across the spark plug's gap. This is called
"Electrical Performance."
The temperature of the spark plug's firing end must be kept
low enough to prevent pre-ignition, but high enough to prevent fouling.
This is called "Thermal Performance", and is determined by the heat range
selected.
It is important to remember that spark plugs do not create
heat, they can only remove heat. The spark plug works as a
heat exchanger by pulling unwanted thermal energy away from the
combustion chamber, and transferring the heat to the engine's cooling
system. The heat range is defined as a plug's ability to dissipate heat. |
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Normal -
Light gray or tan deposits and slight electrode erosion |
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Over Heating - |
An extremely white insulator with small black deposits and premature
electrode erosion.
Symptoms:
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- Loss of power at
high-speed or during heavy load
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Causes:
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- Plug
insufficiently tightened
- Engine
insufficiently cooled
- Ignition
timing too advanced
- Plug heat
range too hot
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Carbon Fouling - |
Dry, soft black carbon on the insulator and electrodes.
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Symptoms:
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- Poor starting
- Misfiring
- Faulty
acceleration
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Causes:
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- Faulty choke -
over rich air/fuel mixture
- Delayed ignition
timing
- Bad ignition
leads
- Plug heat range
too cold
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Oil Fouling -
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Wet,
oily black deposits on the insulator and electrodes
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Symptoms:
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Causes:
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- Wrong piston
rings, cylinders, and valve guides
- Fuel mixture oil
content too high (two-stroke engines)
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Pre-Ignition -
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A melted or burned
center and/or ground electrode, blistered insulator and aluminum or
other metallic deposits on the insulator.
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Symptoms:
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- Loss of power
causing engine damage.
- Pre-ignition
occurs when combustion begins before the timed spark occurs.
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Causes:
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- Plug
insufficiently tightened
- Engine
insufficiently cooled
- Ignition
timing too advanced
- Plug heat
range too hot
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