How to read spark plugs
Thanks to Boonman
 

What is a ”plug chop”?
A plug chop is one way to verify your main jet jetting.  To do a plug chop, make sure the air filter is clean, warm up the motor, shut it off and install new spark plugs.  Start the motor and pin the throttle, running through all gears.  When you wind out the top of top gear, hit the kill switch and pull in the clutch. Keep the throttle pinned!
    Remove the spark plugs (put in the old ones to get home if necessary) and put one in a vise.  Using a hacksaw, cut the threads away from the center electrode with one cut parallel to the electrode and one cut perpendicular to the base of the electrode, so you can see the very base of the center electrode.
 

What will I see?

• If your main jets are spot on, you will see a 2mm tall chocolate-brown colored ring at the base of the electrode.
   

• If the ring is taller than 2mm or darker than a Hershey bar, you’re rich on the mains.
   

• If the ring is smaller than 2mm or lighter, or nonexistent, you’re lean on the mains.
   

• If both plugs aren’t almost identical, one cylinder may be richer or leaner than the other (if leaner then an air leak is likely on that cylinder).

Keep in mind that doing a plug chop is best used as a way to verify your mains. While it’ll cost you a couple bucks for new plugs, the plug chop - if done correctly - is the single best way to find out if your mains are dialed in for your current mods, temperature, and elevation.
 

How do I read my spark plugs?
Aside from doing a plug chop, you can read your spark plugs to get an idea of your overall jetting (to see the base of the center electrode you'll still need to either cut the threads away or use an illuminated magnifying glass).  You’ll need to ride for at least 30 minutes to get a reading, but once done you can find out about your overall jetting, ignition timing, and spark plug heat range.  Be advised that it’s not a good idea to ride around when you know it’s way too lean or rich, and remember that unless the throttle is held at a constant position the jetting indications on the spark plug are an "overall" idea; one circuit can be rich and can mask a lean circuit.

• You set your heat range from the ground strap (this is the piece closest to the piston)
   

• You do all the plug readings for jetting from the base ring (the base ring is what the ground strap is welded to at the end of the threads)
   

• You determine detonation and timing issues from the porcelain The porcelain shows preignition/detonation, it will NOT accurately determine jetting (air/fuel ratios)

 Do NOT base jetting decisions on the porcelain color at the tip!
 

How to determining plug heat range
The ground strap is your window to getting this right. If the "color" of the ground strap "changes" too-close to the ground strap's end, (the end opposite of the base ring), then the heat-range is "too-cold" (heat transfer is too quick to the base ring).  If the "color" of the strap changes near where it is welded to the base-ring, then it means that the plug heat-range is "too-hot" (heat transfer to the base ring is to slow causing the deposits to be burned off the strap completely).  The strap at this point could start working like a "glow-plug", probably resulting in pre-ignition and/or detonation. The properly set heat-range is when the "color" is at the half-way point on the strap.
 

Reading the base ring to determine jetting
The base ring "color" is very close to the color of the piston crown and is used to determine the jetting. You're looking for the soot color to be a nice light to medium brown, (color is always hard to describe) if the color doesn't go all the way around the base ring (at least one full thread turn on the plug) or the color is whitish it is way too lean. If the color goes all the way around, but there is a spotting of heavy dry soot on the top of the color, you are too rich.
 

Read the porcelain to determine detonation/preignition
The first signs of detonation/preignition will be seen on the porcelain down in the plug, It shows up as tiny black or shiny specks of aluminum. Also look very close around the center electrode where the porcelain intersects, this will appear to be melting between the insulator and the electrode. Detonation is caused by the air/fuel mixture exploding rather then burning.  This gives off a sound (a knock), this sound is the result of a shock wave, this wave disrupts the boundary layer of cooler gases that cover the internal parts of the combustion chamber. This causes a very rapid rise in pressure and temperature. The results are holes in the top or sides of the pistons, blown head gaskets, broken rods (all bad stuff), this can also shock the rings from their seal causing oil to form as little spots on the porcelain.
 

   Perfect            Too lean

More reading on the subject...

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