MotoMojo: Spark Plugs—What They Say About Your Engine

May 24, 2016 View Comments by

To check for spark, you can remove a spark plug and crank the engine.Spark plugs can provide invaluable information on the mechanics of your bike. After plugs are run for a while, the tips show the overall condition of the engine, including pistons, rings, valve guides and seals, carburetion or fuel injection, ignition, and spark advance. You can also diagnose a variety of conditions, including detonation, pre-ignition, overheating, incorrect operating temperature, and incorrect heat range due to the wrong plugs.

Determining Current Plug Condition

Before removing plugs, use compressed air (wearing eye protection, of course) to blow dirt away from their base to prevent it from entering the combustion chambers where it can damage the engine. Note which cylinder each plug comes from. If the engine is in good condition, all of the plugs should exhibit equal wear and have a light tan or gray color (lean emission-controlled engines may be almost white). A plug that differs in appearance indicates a problem with a cylinder; but a uniform appearance doesn’t necessarily indicate that all is well, as all plugs can still look the same if the fuel mixture is misadjusted or the engine is worn.

Spark plugs come in different lengths, diameters, thread pitches, and “reach.” Reach is the length of the threaded portion of the shell and it’s crucial to get the correct plug for the engine. Plugs also use platinum, iridium, copper, and other materials to improve performance. Installing a new set of premium plugs can often make a noticeable improvement in overall operation.

“Heat range” indicates how hot the tip gets during operation. Check the owner’s manual for recommended plug numbers. Hotter heat ranges are needed for slow-speed use and a colder plug for high-speed use, such as racing. Generally the standard plug is fine for most street riders.

Match old plugs with new ones for length, type, and number. Check and adjust gaps if necessary, using the specification in the owner’s manual. Slip a piece of 3/8-inch ID rubber hose over the plug’s insulator to guide it into its hole, and start it by hand to prevent cross-threading. Always tighten plugs to the factory recommended torque. Damaged threads can usually be repaired with thread-repair inserts from companies such as Heli-Coil ( and TimeSert® ( Most motorcycle shops and automotive machine shops perform this service.

Diagnosing Plug Condition / Cause and Effect


Misfire is caused by weak or no spark, generally from improper air-fuel mixture or low compression. Causes include worn, fouled, or damaged spark plugs, faulty plug wires, low battery voltage, or a defective coil. A worn-out plug will have rounded, worn electrodes, creating a wide gap. This results in poor engine performance, higher emissions, loss of mileage, and premature ignition component failure.

Over-rich Fuel Mixtures

An over-rich fuel mixture may be indicated by black and fluffy soot on insulators. This may be caused by a leaky injector, faulty EFI sensors, excessive fuel pressure, or an out-of-adjustment carburetor. Rich mixtures cause lower fuel economy, decreased performance, carbon deposits, and damaged catalytic converters.

A wet, fuel-soaked plug tip usually indicates an overly rich fuel mixture or ignition problems. It may be caused by excessive choke use, a carburetor needle and seat remaining open, leaking injectors, or flooding during engine starting. Typically, carbon buildup on plugs is due to extended low-speed riding, rich mixtures, and, in some cases, a plug heat range that’s too cold.


Oil-fouling can show up as an oil-covered tip or thick oily carbon deposits on the insulator. Causes can include worn or damaged cylinders, pistons, rings, and/or valve guides and seals, and occasionally a blocked crankcase vent. Cinder-like heavy ash deposits on the insulator nose can come from oil additives and can lead to power loss and engine damage. If noticed, change the oil and install new plugs.

Cracked Insulators 

Cracked insulators are often caused by mishandling (dropping) or detonation. The cause must be determined before the engine is returned to service. Detonation (pinging or spark knock) is an uncontrolled explosion of air and fuel before the spark ignites it. This is usually caused by excessively high temperatures and loads, and fuel octane that’s too low. Pre-ignition, often confused with detonation, is also an uncontrolled explosion of the fuel, but is caused by glowing spark-plug electrodes, exposed metal burrs, or carbon. Both detonation and pre-ignition can quickly destroy an engine. If the engine is pinging or knocking the load should be reduced and the cause, such as overheating or low octane, determined and corrected immediately.

Melted-center Electrode

A melted center electrode can be caused by over-advanced ignition, excess carbon in combustion chambers, lean air/fuel mixture, or low-octane gas. This is a serious condition that can result in engine damage. A bent electrode usually indicates the plug tip contacted either the valve or piston. Using the wrong plug generally causes this, although over-revving also can do it. Gap bridging is usually found in two-stroke engines, caused by oily deposits that lodge between the electrodes, causing a misfire.


Ken Freund is a former Technical Service Engineer for Champion Spark Plug Co.
Article originally published: October 15, 2014

For additional photos and information visit:


Tours, tankbag maps, tips, and more: subscribe to RoadRUNNER today!

Tags: Categories: Chronicles, Technical Tips