How-To: Spark Plug Check and Replacement

Keeping the spark alive

Spark plugs live a hard life: high pressure, extreme temperatures, and constant explosions. Should we be surprised that the OEs 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 4,000 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.

Spark plug check and replacement
Spark plug check and replacement. Figure A.Photography by Evan Brasfield

Begin by removing any parts, such as the seat, tank and airbox, that prevent access to the plugs. Now, remove the plug wire from one plug at a time. Although it sounds obvious, you don’t want to risk mixing up the plugs and their wiring. Bad things could happen. Once you have access to the plugs, 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 (fig A).

spark plug check
Spark plug check and replacement. Figure B.Photography by Evan Brasfield

Look closely at the electrode and insulator (fig B) which 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, while an extremely white electrode points to possible overheating. Using a brass brush, clean the plug of any deposits. Be careful with the fine electrodes of platinum or other thin tipped plugs. They are extremely hard, to resist wear, but are also quite brittle. Next, measure the gap with a wire thickness gauge. 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. A little effort goes a long way.

Spark plug check and replacement
Spark plug check and replacement. Figure C.Cruiser

When the gap is correct, wipe the threads on a clean rag before applying a little anti-seize 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. Screw in the plug until it is finger tight, and then snug it down with a torque wrench to the factory specification (fig C). This step is vitally important, since some plug failures are associated with incorrectly torqued plugs.

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 4,000 miles.