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4Strokes.com - Tech: How to Read Your Spark Plug

4Strokes.com Tech: How to Read Your Spark Plug

You can tell a lot by reading your spark plug.  Here's how to check your jetting by reading the plug.  Always start with a new plug that is of proper heat range and reach.  Using a plug with an improper heat range or incorrect reach can cause engine damage or poor performance.  Proper torque is essential as an improperly tightened plug can damage the engine.  Before removing any plug, clean the area around it thoroughly to prevent debris from entering the cylinder.  A dry acid brush and an air compressor work great.  Run the engine at least 10 minutes, as a new plug will not color immediately.

To obtain an accurate reading from a new spark plug:

  1. Accelerate at full throttle on a straight
  2. Push the engine stop button and pull the clutch lever in to release the clutch
  3. Coast to a stop
  4. Remove the spark plug
Spark plug Porcelain Insulator
   Center Electrode
Side Electrode

It's best to use a magnifying glass to inspect the spark plug.  The porcelain insulator (1) around the center electrode (2) should appear clean and colorless with a gray ring around the center electrode where it exits the porcelain.  Metallic specks indicate lean jetting that is removing metal from the piston.  Black sooty streaks on the porcelain indicate rich jetting.


Plug Appearance


Normal  White (no color change) with light gray ring  Correct
 Overheating  Extreme white with aluminum specs Lean
Wet Wet or sooty Rich

In addition to improper jetting:

  • A lean condition can be caused by air leaks in the inlet tract or exhaust system, passing of too much air because of the use of the wrong air filter, use of a less-restrictive aftermarket exhaust system, or leaks in the air box.

  • A rich condition can be caused by a plugged or dirty air filter, use of a more-restrictive aftermarket exhaust system, a clogged spark arrester, or excessive oil on the air cleaner.  Excessive smoking may occur with this condition.

A good article from our Forums:

Reading spark plugs isn't too hard to do, but I've read various understandings of how to read spark plugs and it seems there's different schools of thought on this. I always use a lighted magnifying glass of 5X to 10X to better see the tiny deposits. When checking your spark plug, always use a good used spark plug, making sure it's not worn out. A worn out spark plug will have a worn or rounded center electrode and or side electrode. New spark plugs are hard to read accurately for mixture checks, but if you have to use a new spark plug, then make sure to put at least 15 to 20 hard minutes on it before attempting to get a reading. I do my spark plug readings from the base ring at the bottom of the threaded body. The 'L' shaped grounded side electrode that's welded to the base ring will clue you into the heat range as this is the closest part to the piston. The porcelain will clue you into timing and preignition/detonation issues while the base ring will clue you into the mixture. Many people I've talked to and some of the articles I've read simply look at the porcelain color and use that as their sole guide for a proper air/fuel mixture, but I don't agree with that all together and would not recommend basing your jetting decisions solely on the color of your spark plugs porcelain.

Center/Ground Electrode

You want to first carefully look at the grounded side electrode to determine if you've got the proper heat range for your spark plug and this is done by closely examining the color change of the side electrode. If the color of the electrode changes near its end where it sits over the center electrode, then the spark plug heat range is too cold and what you're seeing is the color changing due to the heat transferring too quickly. If the color of the side electrode changes color near where it's welded to the base ring, then your spark plug is too hot and what you're seeing is a slower heat transfer from the side electrode to the base ring, resulting in preignition/detonation issues and most of the deposits will be burned off. Ideally, you want the side electrode of your spark plug to change color at about the half way point, about where it makes it's 'L' shaped bend.

Base Ring

The second thing you want to check is your spark plugs base ring and this will clue you into how your bike is jetted. The color of the base ring itself is close in color to the crown of the piston and what you're looking for here is a nice light to medium brown color all the way around the base ring. If you're seeing a chalky whitish or light grayish color or the color doesn't uniformly go all the way around the ring, then you are running too lean. If the color does go all the way around the ring, but you see dark colored soft dry soot that's heavily spotted on top of the base ring color, then you're running too rich and or possibly have a spark plug with too cold of a heat range. The presence of wet oil or ash deposits is a tell tale sign of possible engine problems such as valve stem or valve guide wear or worn out piston rings, etc, so don't confuse this with a rich mixture that leaves your spark plug carbon fouled.

Porcelain Color

The third thing to check is the porcelain color and this will clue you into preignition/detonation issues. What you're looking for are tiny specs of aluminum on the porcelain, which can be either black or shiny. If the tip of the insulator appears melted, then this is yet another clue to a pre-ignition/detonation problem. The detonation is caused by the air/fuel mixture exploding instead of burning and you may hear the resulting knocking sound from this, particularly when the engine is under a load. The knocking sound heard is actually a shock wave that's disrupting the boundary layer of cooler gasses that cover the internal parts of the combustion chamber, resulting in incomplete combustion. This rapid rise in pressure and temperature exerts extreme force on engine components and can do very bad things such as crack your engines head, crack or put holes in your piston, blow head gaskets, break your connecting rod, damage bearings, seals, etc. This is why you should not base all your jetting decisions on just the porcelain color alone because the porcelain color doesn't tell the whole story. Although the porcelain and base ring colors are similar, the porcelain usually appears lighter in color when compared to the base ring.

Posted by qadsan

A few good links on spark plugs:

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