On most motorcycles, the spark plug wires connect the ignition coil to the spark plugs (in some cases the coil may be integral to the spark plug wire). There is one wire for each spark plug. They may also be called spark plug leads, as they often do not have a core of copper wire, but may be carbon or another material. Parts of a spark plug wire are the core, which conducts an electrical charge, the outer jacket of insulation, the metal terminals at each end wire that connect the wire to the coil and the spark plug, and the boots that cover the terminals.
Spark plug wires transmit a very high electrical charge – thousands of volts – generated by the coil to the spark plug. Modern spark plug wires are also designed to suppress electro-magnetic interference (EMI) or radio frequency interference (RFI) that can interfere with the ignition system or electronics on the motorcycle.
Spark plug wire terminals, also known as connectors, are metal terminals at each end of the wire that make a physical connection to the coil on one end and to the spark plug on the other end.
There is no pre-determined life span for spark plug wires. The life span of motorcycle spark plug wires is determined by a number of factors:
Bad spark plug wires can limit or interrupt the voltage transmitted from the coil to the spark plug. Signs that motorcycle spark plug wires should be inspected include diminished fuel economy or uneven engine performance. If the spark plug wire core is damaged, transmission of voltage to the spark plug may be occasionally interrupted and engine performance will sometimes feel “rough.” If the engine misses or runs roughly in wet weather, the spark plug wire insulation could be cracked or damaged. These symptoms could also be caused by a bad coil, a poor ground, or an issue elsewhere in the motorcycle electrical system. If you’re unsure if your spark wires are faulty, it may be possible to have a Harley-Davidson® dealer scan the engine management system for a trouble code that indicates a potential fault.
It’s recommended to consult your motorcycle operator manual or service manual for specific instructions on how to remove a spark plug wire from your motorcycle. Usually removing the wire is as easy as disconnecting each end of the wire at the spark plug and the coil or ignition system. However, over time heat and age can cause the boots to stick to the coil and especially to the spark plug insulator. Never pull directly on the wire; instead always pull on the boot. A spark plug wire puller is a useful tool for removing a stuck boot. It has soft and wide jaws that will grip the boot and allow you to twist and pull on the boot to break it free.
The routing of the spark plug wire from the coil to the spark plug can be important, and the wire may be supported by a clip or tie to keep it from touching other parts of the bike. Take a quick picture of the wire routing before removing it so you can reinstall correctly.
Reaching the spark plugs may require removing the motorcycle seat, the fuel tank or other components. Harley-Davidson recommends replacing the spark plugs in Milwaukee-Eight™® engines every two years or every 30,000 mi (48,000 km), whichever comes first.
It’s recommended to consult your motorcycle operator manual or service manual for specific instructions for your bike.
Solid core spark plug wires typically utilize a stainless steel or copper wire core, which conducts electrical current very well. For that reason, solid core wires have lower resistance than other wires to get maximum energy to the plugs. However, a solid core offers very little electro-magnetic interference (EMI) or radio frequency interference (RFI) suppression, so they’re not suited for electronic ignition systems or motorcycles with sensitive communications equipment. Older motorcycles with a carburetor and magneto or points-style ignition benefit from the ultra-low resistance of solid core wires.
Motorcycle spark plug wires should be replaced, rather than repaired. Your Harley-Davidson® dealer is the best source for replacement spark plug wires. The original equipment-style wires sold by the motorcycle manufacturer through its dealer will be the correct length, material, and boot shape and have the correct resistance, which can be important to maintaining performance with modern ignition systems. Harley-Davidson dealers offer both standard original equipment spark plug wires and Screamin’ Eagle® Phat Spark Plug Wires for many models.
Working on a vintage or custom motorcycle may require building your own spark plug wires. Seek expert advice on the type of spark plug wire that will work best with your motorcycle ignition system.
The spark plug wire terminals are often crimped on the end of the wire after a small section of the insulation has been trimmed away. When making up a set of custom spark plug wires, you should use a special crimping tool to do this task correctly. A quality crimping tool may have interchangeable jaws for different style and size terminals. Start by lightly crimping the terminal just enough to hold it in place, and then check to make sure that the conducting wire is correctly positioned under the terminal – once you finish the crimp it will not be possible to do it over. Then use the crimping tool to secure the terminal.
The boots on each end of the spark plug wire cover the terminal to protect it from dirt and moisture. Boots may be straight in line with the wire, or at an angle – right angle/90 degrees is typical. Straight boots can often be pushed over the terminal after it has been installed on the wire. It can help to lubricate the boot with a little dielectric grease or soapy water. It’s usually easier to install right-angle boots if they are placed over the wire before the terminal is crimped down. After crimping the terminal, slide the boot down the wire and into place. Again, a little motorcycle lubricant can help to move it into place
Harley-Davidson offers both replacement 8mm Original Equipment-style spark plug wires and 10mm Screamin’ Eagle® Phat Spark Plug Wires for many Harley-Davidson® models. Both are available from an authorized Harley-Davidson® dealer and 10mm Screamin’ Eagle® Phat Spark Plug Wires are available on H-D.com.