How To Replace Motorcycle Spark Plugs

Yearly replacement of spark plugs is a maintenance practice that can help improve performance and avoid breakdowns and unnecessary repairs. A correct "reading" of a used spark plug's condition can provide valuable information about performance and potential issues in your bike's main systems.

A well-functioning, used spark plug may have some tan or gray deposits on the firing tip but no signs of abnormal wear or erosion on the gap electrodes; a read of this spark plug suggests normal operating conditions.

Replace spark plugs with one or more of these conditions: sooty, carbon-fouled, or oily deposits; melted or eroded electrodes; deposits between the electrodes; and breakage or blistering on the ceramic surface. See step 6 of this task for details.

As always, be sure to check your service manual for specific maintenance information.

Prerequisite Tasks:

Remove (or reposition) the gas tank. NOTE: Many motorcycles require complete gas tank removal to access the spark plugs, but on some bikes, spark plugs can be accessed by repositioning the gas tank. However, complete gas tank removal may help prevent hazardous contamination of open spark plug holes and damage to surrounding surfaces.

Associated Task:

While the fuel tank is removed, you can: inspect the gas tank and repair leaks, damage, interior rust, and excessive wear on the tank mounting bracket and rubber mounts and perform tasks on the carburetor and engine valves.

• Maintenance work stand
• Shop towels
• Socket wrench
• Spark plug socket with rubber insert
• Wire feeler gauge
• Spark plug gapping tool
• Anti-seize compound
• Torque wrench

WARNING: Gasoline is extremely flammable and explosive. This task must be performed in a well-ventilated area away from open flames, including appliance pilot lights.

CAUTION: To prevent engine damage DO NOT allow dirt and debris to fall into open spark plug holes. Before removing spark plugs, always remove loose debris and carefully clean the cylinder head cover and any other adjacent areas.

1. Stabilize the motorcycle with a maintenance stand per manufacturer's recommendation.
2. Per the manufacturer's service manual, remove (or reposition) the fuel tank for spark plug access.
3. Remove debris and clean the cylinder head cover and any other adjacent areas that could allow debris, dirt, and dust to fall into open spark plug holes. CAUTION: To prevent damage, do not grasp or twist spark plug wires while removing the spark plug cap.
4. Disconnect the spark plug cap:
   - Slowly twist the spark plug cap in both directions to loosen the cap and the cap's seal to the cylinder head cover.
   - Pull up on the cap, and remove it from the spark plug.
5. Remove the spark plug:
   - Before installing the socket wrench handle, put a spark plug socket with a rubber insert on the spark plug and slowly twist it in both directions to seat the spark plug socket.
   - Attach the wrench handle to the socket, and remove the spark plug.
   - Label the spark plug with its cylinder number.
6. Inspect the spark plug, and discard any spark plug with one or more of these conditions:
   - Sooty deposits.
   - Carbon-fouled deposits on the firing end of the plug.
   - Oily deposits on the insulator tip or the firing end of the plug.
   - Melted or eroded electrodes.
   - Deposits between the electrodes.
   - Breakage or blistering on the porcelain insulator surface.

NOTE: Some replacement spark plugs come with a terminal nut that fits both terminal and threaded stud connection types. If the terminal nut is not required, twist the nut off to remove it.

1. Set the spark plug gap:
   - Put the wire feeler gauge between the spark plug's center and side electrodes, and pull the wire through the electrode gap. A slight drag on the feeler gauge wire is a preliminary indicator of correct gap width.
   - If there is no drag on the wire or if the wire will not pass through the gap, use a gaping tool to slightly bend the side electrode.
   - Set the electrode gap to the manufacturer's specification using the wire feeler gauge and gaping tool.

1. Apply a light coat of anti-seize compound to the spark plug threads.
   CAUTION: To prevent damage to the spark plug, do not force the spark plug into the internal threads. If more than light pressure is required, remove the spark plug and try again.
2. Install the spark plug:
   - Use light hand pressure to screw the spark plug in, and then apply hand-tight - pressure to seat it.
   - Use a torque wrench to tighten the spark plug to the manufacturer's specification.
   - Do not over tighten.
3. Put the spark plug cap back on the plug and slowly twist it in both directions to install:
   - Make sure that the cap is in contact with the spark plug.
   - Make sure that the cap makes a sealed contact with the cylinder head cover.
4. Install or reposition the gas tank per the service manual instructions.

- End of Procedure -

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