Servicing Spark Plugs

It's easy, quick and can save you money. Story and photos by Evans Brasfield.

Spark plugs live a hard life: high pressure, extreme temperatures, and constant explosions. Should we be surprised that the motorcycle manufacturers expect us to check the conditions of our bike's plugs at every service? If you look at your manual, that should be just about every 4000 miles. Fortunately, the plugs don't often need replacing, just a quick brush off. Still, don't let spark plugs' hardiness lull you into a false sense of confidence. Few things can wreak havoc on your engine's performance like a fouled plug. Luckily, checking and replacing your plugs is an easy job, requiring little more than a deep socket of the appropriate size, a torque wrench, a brass brush, and a wire spark plug gauge.

Begin your spark-plug service by removing any parts, such as the seat, tank and airbox, that prevent access to the plugs. Many bikes have none of these obstacles. Now, remove the plug wire from one plug at a time. Although it sounds obvious, you don't want to risk mixing up the wiring. Bad things could happen if you connect the wire to the plug for the wrong cylinder. Once you have access to the plugs, but before you remove them, blow the plug wells out with compressed air (or a can of Dust Off from a photo or computer store). You'd be surprised how frequently sand or pebbles pop out of hiding places in there, and having such debris drop into the cylinder could spell disaster.

Look closely at the electrode (the metal tip that extends up from the middle of the plug) and insulator (the white ceramic part surrounding the electrode). the insulator should have a light tan or grey color. If the insulator is cracked, replace the plug. If the plug has a heavy build-up of any kind, take it to a local shop to get a qualified mechanic's opinion. As a rule of thumb, a dark, sooty plug points to a rich mixture (dirty air cleaner, overly rich jetting, or some other cause of too much gas or too little air in the intake misture, while an extremely white electrode points to possible overheating, possible because of a lean mixture. Using a brass brush, clean the plug of any deposits. Be careful with the fine electrodes of platinum or other fine-wire-tipped plugs. These electrodes are extremely hard, to resist wear, but are also quite brittle.

Next, measure the gap with a wire thickness gauge. If the wire feeler oes through the gap with slight resistance, you have found the gap's size. If the gap is too narrow, use the gapping tool usually attached to a set of wire gauges to carefully bend the side electrode outward slight. If it's too wide, do the opposite. A little effort goes a long way.

When the gap is correct, wipe the threads on a clean rag before applying a little anti-seize lubricant to them. Carefully insert the plug into the plug hole. Rotate the plug counter clockwise until you feel the threads drop into synch. Then, using only your fingers, rotate it clockwise to engage the threads. Cross-threading a spark plug can be a time-consuming and expensive pain, so screw in the plug in until it is finger tight (at least a few full turns), and then snug it down with a torque wrench to the factory specification. This step is vitally important, since some plug failures are associated with incorrectly torqued plugs, and over-tightening can strip out the plug-hole threads.

Before reassembling your entire bike, start the engine to make sure all of the cylinders are firing correctly. If anything sounds amiss, check all of the plug connections. Restart the engine, and if the problem remains, check to make sure all of the wires are connected to the correct cylinders. When everything sounds right, button up your bike and go ride another 4000 miles. Evans Brasfield

Evans Brasfield is currently writing books about motorcycle maintenance and modification for Motorbooks International. He welcomes your electrifying comments at his web site:

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Examine the electrode and internal insulator and check the gap between the metal electrodes.
Before removing the spark plug, it is a good idea to blow out its well with compressed air to make sure no debris in the area falls into the cylinder as the plug is extracted. If they are going to be out for a while or you are going other work, cover the holes with rags.
Using a torque wrench gets these critical components seated properly without the danger of stripped threads.