Spark Plugs

The spark plug has one simple, but vital function – to create a spark that ignites the fuel and air in the combustion chamber, causing a controlled explosion that moves the piston.


A spark plug has a center electrode, a ceramic insulator, a metal casing or shell, and a side or ground electrode. The center electrode is a thick piece of metal wire inside the plug, conducting electricity from the ignition cable hooked to the end of the plug to the electrode gap at the other end in the combustion chamber. The insulator is a ceramic casing that surrounds most of the center electrode. The metal casing is a hexagon-shaped shell threaded on one end to allow the spark plug to be installed into a tapped socket in the engine cylinder head. The side or ground electrode is connected to the metal shell and extends toward the center electrode. The tips of the two electrodes are about 0.020 - 0.080 inch apart, creating the gap for the spark to jump across.


The heat range of a spark plug refers to thermal characteristics. It measures the amount of time it takes heat to be removed from the tip of the plug and transferred to the engine cylinder head. If the plug tip temperature is too cold when it sparks, carbon, oil, and combustion products can cause the plug to foul. If the plug tip temperature is too hot, you’ll get pre-ignition -- the center electrode burns, and you may damage the piston. Heat range is altered by changing the length of the insulator nose. In short, a long nose results in a hot plug, and short nose is cold.


Some people think it’s critically necessary, others think it’s useless. Indexing a spark plug means installing it so that the open area of the spark gap (the part not blocked by the ground electrode) faces the center of the combustion chamber, towards the intake valve. The belief is that it optimizes the exposure of the fuel-air mixture to the spark. It’s done by marking the location of the gap on the outside of the plug, installing it, and noting the direction the mark faces. Then you have to remove the plug and reinstall it with the right number of washers so that the mark faces in the right direction. You have to redo the process every time you install a new spark plug, since the threads are unlikely to line up the same way from plug to plug.


The spark plug cap is the fitting at the end of the cable that connects to the plug. Most of us still have the same sparkplug cap the bike came with. But if yours broke or wore out, or you’re one of those riders that has to change everything possible, you have a few choices. The best criteria to use when deciding is to pick the spark plug cap that best matches the natural angle between the wire and the plug itself.


Chances are good that, if you bought the exact plug your bike manufacturer calls for, your spark plug is already gapped properly – BUT it’s best to check anyway.

Make sure the gap is what your manual calls for. If you don’t have a manual, ask when you buy the plug. A gap that’s too small gives a short and weak. If the gap is too wide, there may not be enough power to bridge the gap and generate a proper spark.

Only use the wire-type of gauge to measure spark plug gap. They’re far easier to use and far more accurate than the flat feeler gauges.

If you have to adjust the gap, use the proper tool (often found built in to your gauge). Never hold the spark plug by the ceramic neck when adjusting the gap – the leverage can easily crack or break it.


Spark plugs are disposable -- take it out and toss it. But spark plugs are also a good way to check and diagnose your dirt bike’s engine.

Above is what a normal plug, operating under normal conditions, should look like. A light grey, light tan, or white color means you have the proper heat range and jetting.

The above spark plug is basically just worn out. The color is normal, but you may have noticed some hard starting in damp or cold weather and – if you keep track of that sort of thing – you might have been burning more fuel than usual. Good thing you have that new plug ready to install!

Ouch! Something has smacked your spark plug causing physical damage – either something is in the combustion chamber that shouldn’t be, or your spark plug is too long. First double-check that you have the right length of spark plug -- if you don’t, you’ll solve the problem by buying the correct length. If the plug is the right length, you’ll have to remove whatever foreign material is in the combustion chamber – excessive carbon buildup is often a culprit.

A chipped or cracked insulator tip, like the one above, can result from severe detonation. The wrong gap settings can also cause cracking and chipping. Detonation can lead to piston damage, so deal with it quickly. Check your fuel octane, ignition timing, and your plug’s heat range. If your dirt bike is equipped with EGR or a knock sensor, make sure they’re operating properly.

At first glance, the above overheated plug doesn’t look like much is wrong with it. Check for a chalky look or feel, white (possibly blistered) ceramic, and very little or no deposits. The electrode will also have worn down faster than usual. Check your ignition timing, air/fuel mixture (is it too lean?), intake manifold vacuum leaks, sticking valves, and the spark plug’s heat range. On liquid-cooled engines, check the coolant level and inspect for clogs.

Those light brown bits of stuff on the end of your plug may look a little like rust, but they’re ash deposits. Check for worn valve guides or seals. If you’re using fuel additives, stop using them and check the plug to see if that’s the cause. Sometimes, changing your brand of fuel can help.

When your spark plug is covered in oil like the one above, something is leaking. Oil is sneaking into the combustion chamber, probably via worn valve guides, or piston rings. You’ll notice hard starts, misfires, and hesitation. The only solution is too fix the mechanical problem that’s allowing the oil in.

Initial pre-ignition sounds like something involving a rocket launch. You can identify it by a melted electrode. Check your spark plug’s heat range, too lean a fuel mixture, timing that’s advanced too far, or accumulations that could be causing a hot spot inside the combustion chamber.
If the insulator is destroyed, like the plug shone above, and the electrodes are melted or missing, it could be sustained pre-ignition. Check for too lean a mixture, the wrong heat range for your spark plug, over-advanced timing, hot spots in the combustion chamber (often caused by deposit accumulation).

Little bits of contaminants on the ceramic is often referred to as splashed deposits. Most likely a dirty carburetor bore or air intake -- or a dirty or faulty injector if’n ya gots one. Clean the carburetor and choke assembly, or clean or replace the injectors before you bother putting a new plug in.

If you’re running too rich a mixture, have a weakness in your ignition system, or a spark plug with the wrong heat range, you may get a carbon fouled plug, like the one above. It can cause hesitation, misfiring, and hard shifting. You’ll identify it by dry, black, soot. Check the heat range of your plug first, then examine the choke and make sure it’s working. Take a look for a clogged air element, worn ignition points, and a float level that’s set too high. If you have injectors you may have a clog, a fault in the cold start circuit, or the wrong fuel pressure.

If the insulator has a glazed (high speed glazing), yellowish appearance, it indicates that the combustion chamber temperature has gone up suddenly during hard acceleration, causing normal deposits to melt and form a conductive coating. You’ll notice misfiring at higher speeds. Try using a spark plug with a lower temperature range.

If your bike just dies one day, and you pull the plug to find heavy deposits in the gap – you have gap bridging which simply shorts out the plug and prevents a spark. It’s caused by combustion deposits knocked loose and lodging between the electrodes. Deposits accumulating on the side electrode may also melt when the engine is put under a sudden, heavy load, bridging the gap. Replace the plug.