reading used plugs
http://densoiridium.com/howtoreadusedplugs.php
Reading a Spark Plug
NASCAR engine tuners love spark plugs they can "read". The discoloration on the insulator after a few laps helps them understand how the electrical elements of the engine are performing.
Not all plugs allow you to "read" them. The corona, a brown deposit on the insulator, is the glow that appears above the spark plug housing around the base of the insulator. The appearance of a corona is not detrimental to the operation of the spark plug. Contrary to common belief, this condition is caused by electrical stress in the air adjacent to the insulator. Often, the corona stain is mistaken for spark plug gas leakage, but again, this does not affect spark plug performance.
The insulator is ceramic and can be broken, although this is a rare occurrence. Typically, the most common ways the insulator becomes cracked is when the installation is done at the wrong angle, the incorrect torque is used or the removal is forced. (See Installation Guide)
Our primary concern is that you use the appropriate tools and torque during installation. When the insulator is cracked during installation, this can cause a short circuit which will result in irregular idling and misfires during acceleration. (See Common Engine Problems)
Diagnosis of a Used Spark Plug
If you're seeing any of the following, you probably don't have the manufacturer's recommended spark plug installed. But the symptoms are separate from the causes and all the components in an ignition system, no matter how small, will leave some signs on the spark plugs over time if they're not in harmony. This guide can provide insight to the engine's performance.
Normal
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Light grey or tan deposits and slight electrode erosion
Carbon Fouling
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Dry, soft black carbon on the insulator and electrodes.
Symptoms:
Poor starting
Misfiring
Faulty acceleration
Causes:
Faulty choke - over rich air/fuel mixture
Delayed ignition timing
Bad ignition leads
Plug heat range too cold
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
Symptoms:
Loss of power causing engine damage
Pre-ignition occurs when combustion begins before the timed spark occurs.
Causes:
Plug insufficiently tightened
Engine insufficiently cooled
Ignition timing too advanced
Plug heat range too hot
Over Heating
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An extremely white insulator with small black deposits and premature electrode erosion.
Symptoms:
Loss of power at high-speed or during heavy load
Causes:
Plug insufficiently tightened
Engine insufficiently cooled
Ignition timing too advanced
Plug heat range too hot
Mechanical Damage
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Bent electrode and a broken insulator, dents often present on electrode
Symptoms:
Misfiring
Causes:
Plug nose is too long for engine head
Foreign object (bolt/nut) in combustion chamber
Oil Fouling
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Wet, oily black deposits on the insulator and electrodes
Symptoms:
Poor starting
Misfiring
Causes:
Wrong piston rings, cylinders, and valve guides
Fuel mixture oil content too high (two-stroke engines)
Broken Insulator
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Insulator is cracked or split
Symptoms:
Misfiring
Causes:
Severe detonation
Incorrect tool/torque applied during installation or removal
Careless gap setting
Torched Seat
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Melted in the thread and seat area of the plug housing
Symptoms:
Loss of power causing engine damage
Causes:
Plug insufficiently tightened
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