Motorcycle spark plugs: colour diagnosis and replacement

The spark plug: your combustion witness

The spark plug is the only component you can remove and inspect to get a direct idea of what's happening inside the combustion chamber. Its colour, condition, and deposits tell a story. Learn to read it.

Colour diagnosis

Light brown to chocolate brown: all good. Combustion is correct, the air-fuel mixture is optimal. This is the colour you want to see.

A single plug that looks different from the others on a multi-cylinder engine points to a localised problem on that cylinder (injector, compression, valve).

Choosing the right plug

Every engine has a specific plug reference. The two major brands are NGK and Denso. The reference is in the owner's manual or on the original plug.

Never change the heat range without reason. A plug that's too cold on a stock engine causes fouling. A plug that's too hot causes pre-ignition.

Replacement procedure

1. Engine cold — aluminium heads expand when hot, and removing a plug from a hot engine risks damaging the threads
2. Blow around the plug well with compressed air to prevent debris from falling into the cylinder
3. Unscrew the old plug with a spark plug socket (16 mm or 18 mm depending on model)
4. Inspect the threads of the well — clean, no deposits
5. Thread the new plug in by hand first, to avoid cross-threading
6. Torque to spec: 12-18 Nm depending on head material (aluminium = 12 Nm, cast iron = 18 Nm). Without a torque wrench: hand-tight to contact, then turn 1/4 turn (new plug with washer) or 1/12 turn (tapered plug)
7. Reconnect the plug cap firmly

Replacement intervals

• Standard plugs (nickel): every 10,000 km
• Iridium / platinum plugs: every 20,000 km

Intervals vary by model. A twin that sees a lot of city riding will wear plugs faster than a four-cylinder on the motorway.

The socket wrench trap

Never torque a spark plug with a standard socket wrench without torque control. The cylinder head threads are aluminium on most modern bikes — stripped threads mean a Helicoil insert or a new head. The cost of a torque wrench (30-50 EUR) is nothing compared to the cost of a cylinder head.

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