How to Read a 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

It is 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.

What does a spark plug look like for different carburetor mixture conditions:

  • Optimal – White or no color change with light gray ring
  • Lean – Extreme white with aluminum specs (overheating)
  • Rich – Wet or sooty

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.

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