Cleaning Spark Plugs

Question:

  • What is a fouled spark plug? Should you clean old, dirty or fouled spark plugs and reinstall them?
  • Answer:

    Spark plugs become fouled or dirty when the insulator nose at the firing tip becomes coated with a foreign substance such as fuel, oil or carbon. This coating makes it harder for the spark to bridge the gap and fire normally. When the firing tip is dirty, the voltage or spark, travels along the insulator nose, back down into the metal shell and grounds out. It can no longer efficiently burn the fuel.

    What causes spark plug fouling?

    Many factors can contribute to spark plug fouling.

  • The air to fuel ratio may be too rich as a result of incorrect carburetor adjustment, a dirty air cleaner element or a poorly performing fuel injection system, coating the plugs with gasoline.
  • Worn piston rings or valve seals may allow too much oil to leak into the combustion chamber, fouling the plugs with oil.
  • Improperly functioning ignition or charging system.
  • Prolonged idling or continuous low-speed driving may be preventing the plug from reaching its optimum operating temperature leading to carbon build up.
  • Installing a spark plug with too cold of a heat range.

  • Once a spark plug is fouled, it will not provide adequate voltage to the firing tip and the cylinder will not fire properly.



    The high energy ignition systems of most modern motorcycles have very small tolerances. All components of the electrical system must work within a specified range for the machine to operate well.

    Cleaning spark plugs may have been commonplace 30 or 50 years ago, but no longer. Remember that spark plugs are a maintenance item. They wear out just like oil, chains and sprockets.

    In most cases, the spark plug cannot be cleaned sufficiently to restore normal operation. They may last a day or a week, but they will never be as efficient as new. Once your spark plugs have been fouled by oil, carbon or unburnt gasoline they need to be replaced.

    In our experience fouled spark plugs are the number one cause of misdiagnoses, confusion and aggravation faced by do it yourself motorcycle mechanics. Just because you replaced the plugs 2 weeks ago does not mean that they aren’t the problem.



    There will always be instances when you have to attempt cleaning spark plugs. Such as when you are broken down on the side of the road or the plugs for your vintage machine are back ordered at the height of riding season.

    NGK Spark Plugs' website states that if you are determined to clean your plugs, follow the following steps:

  • If the firing end is wet, make sure you clean the spark plug with a quick drying cleaner. (Examples: contact cleaner or brake cleaner).
  • Sand blast the spark plug using low air pressure and use a dry compound.
  • Completely blow all the sand from the spark plug.
  • Using a wire brush, clean the threads and re-gap.


  • There are many people, webpages and blogs that will tell you that cleaning spark plugs is cheap and easy. There are ebooks and reports available for sale. Even spark plug cleaning machines or systems you can purchase.

    It is our opinion that cleaning spark plugs may be fine for your lawn mower or 1950s Chevy, but not for a European or Japanese sportbike.


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