Shovelhead Spark Plugs and Wires

Spark plug R&R is relatively straightforward. Always examine your used spark plugs upon removal. Reading the spark plug condition will help you to better understand how your motorcycle is running. It can even help you to diagnose an existing issue. If your spark plugs appear very grey or white in color, it is an indication that the motorcycle is running lean. This could mean that your fuel mixture is incorrect or it can be caused from an intake leak.

If the spark plugs appear black and sooty, it is an indication that the motorcycle is running rich. This can be caused by an incorrect fuel mixture or it can be caused by leaving the choke engaged for too long depending on what carburetor is installed on the motorcycle.

If spark plugs appear to be encrusted in oil, it is an indication that oil is getting past the engine’s piston rings or valve seals.

Most spark plugs are available in different heat ranges. If you live where the climate is typically very warm, you may want to experiment with different heat range spark plugs. It is vitally important that spark plugs are gapped correctly prior to installation. Always refer to the factory service manual for your correct year and model Harley-Davidson for the correct gap measurement. Never assume that new spark plugs are pre-gapped correctly for your Harley. Even if you purchase spark plugs from a Harley Davidson dealership.

It is always a good idea to put a light amount of anti-seize on the threads of your spark plugs prior to installation. This is helpful so that the threads in the front and rear heads stay in perfect condition for many years, even after many spark plug changes. Always start threading spark plugs into the heads by hand and never use any type of power tools in this area of service.

Inspect spark plug wire conditions at every service or whenever spark plugs are replaced. Dielectric grease should also be used to help protect the connections from the elements. Address your spark plugs and spark plug wires at every general service.