4 min read · updated 2026-10-14

Checking your spark plug

A worn or dirty plug makes starting hard and pickup weak. The plug also tells you how the engine is running.

What you need

The plug spanner from the factory toolkit
A clean rag and a small wire brush
Your owner's manual for the correct plug type and gap

Steps

Work on a cold engine. A hot cylinder head can burn you and damage the threads.
Pull the plug cap off by its cap, not by the wire. Brush or blow away dirt around the plug so nothing falls into the engine.
Unscrew the plug with the spanner and look at the tip.
Light brown or grey: healthy. Dry black soot: a choked air filter or a fuel mixture problem. Wet with oil: see a mechanic.
Clean light deposits with the wire brush. Check the gap against the manual, or ask a mechanic to set it.
Screw the plug back in by hand first, then snug it with the spanner. Push the cap on until it clicks.

Common mistakes

Starting the thread with the spanner. A cross-threaded plug can ruin the cylinder head.
Over-tightening. Snug is enough.
Fitting any plug that fits the hole. The heat range and type must match the manual.

When to go to a mechanic

An oily or white, blistered plug tip.
A cracked white insulator, or a plug that will not come out easily.

This is general guidance for typical commuter motorcycles. Your owner's manual is the final word on pressures, intervals and oil grades for your bike. If you are unsure, ask a trusted mechanic.