How Do You Read A Motorcycle Spark Plug?

Cycle World Technical Editor Kevin Cameron answers your motorcycle engineering and mechanical questions

Today’s motorcycles take care of the fuel/air ratio automatically. They have an oxygen sensor in the exhaust pipe, which continuously monitors the state of combustion, and the ECU on your engine corrects this. It wasn’t always so.

In the days of carburetors, if the sun went behind a cloud or a cold front moved in, or the barometer went up or down, you had to do the compensating yourself because, well, the human was the active system; the carburetor was the passive system.

After every practice at the races, a mechanic would remove the spark plugs from the engine and lay them out in order for inspection. Using a small magnifier, often with a light in it, the tuner would look in detail at the end of the spark plug that is in the combustion chamber.

When you look at a spark plug, you have to make a distinction: Is this a spark plug that has done 5,000 miles on the street? Or is this a spark plug that was put into this engine brand-new and has now run a seven-lap practice? The spark plug does not “clean up” from being dark and dirty. That’s why in racing people were constantly putting new spark plugs into their engines. Installing a used spark plug to evaluate your combustion would be like using a red handkerchief to find out if you had a bloody nose.

Down some distance from the tip of the insulator, tuners wanted to see a dark ring that indicated the mixture was not yet too lean. If there is some free carbon in the combustion gas, you will see that carbon forming a ring at some point along the temperature gradient. The farther the ring is toward the hot end of the spark plug, the richer your fuel mixture. The farther it is to the other end of the plug, the leaner your fuel mixture. That’s why all the peering went on. That’s the basic story on spark plugs.

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for Cycle magazine and, since 1992, for Cycle World. Kevin’s unparalleled experience and knowledge of the sport were—and continue to be—prompted by a lifetime of curiosity. His willingness to share that information with anyone who is willing to listen is likewise unique.

Kevin’s greatest strength lies in his ability to present complex subjects in simple terms with clarity and, often, humor. In this video series, shot in his home shop, Kevin draws upon his vast historical references to address modern-day questions. As Kevin has written, “Emotions bring us to engineering, but engineering then becomes a special way of confronting reality.”