Remove, check, gap and fit a spark plug

The spark plug lights the fuel in the cylinder. A dirty, worn or wrong plug causes hard starting, misfires and poor fuel use. Check it at each service and whenever the engine starts or runs badly. The old plug also tells you how the engine is running.

Safety

• Work on a cold engine. The head and exhaust burn, and a hot aluminium thread strips more easily.
• Key OFF.
• Do not let dirt fall into the plug hole.

You need

• The correct plug: same thread size, reach (thread length) and heat range as the original. K-Light 125: D7RTC or equivalent. Other bikes: the plug code in your manual.
• Iridium and platinum plugs: do not brush the centre electrode and do not bend it; many should not be re-gapped. Follow the plug maker's guide.

Steps

1. Pull the plug cap straight off. Do not pull the lead.
2. Blow or brush dirt away from around the plug.
3. Undo the plug with the plug spanner, keeping the spanner straight.
4. Look at the plug. Check for a cracked insulator, worn or rounded electrodes, and the colour of the deposits. K-Light 125 manual: light grey is normal; pale white points to a lean mixture or ignition fault; wet or black carbon points to a rich mixture. See Read a spark plug.
5. Replace the plug if the insulator is cracked, the electrodes are eroded, or it is oily or heavily fouled.
6. Light dry carbon on a plain plug can be cleaned off with a soft wire brush. Blow away all loose bits.
7. Check the gap with a wire gauge. K-Light 125: 0.7 ± 0.1 mm. On a new plain plug, bend only the side electrode to set it. The K-Light 125 manual advises replacing a used plug whose gap is out of range (PDF page 89).
8. Start the plug in its hole by hand and turn it until it seats. If it is hard to turn, stop: it may be cross-threaded.
9. Tighten with the torque wrench. K-Light 125: 15 N·m. Other bikes: use your manual's value, or the plug maker's guide.
10. Push the cap on until it clicks. Check the cap rubber is not cracked.

Check your work

• The engine starts and idles smoothly.
• The cap is fully on and does not move when tugged gently.

If it did not fix the problem

• New plug fouls again quickly: find the cause through Read a spark plug.
• Still no start: Check for a spark safely.
• Misfire continues: Test the ignition coil and plug cap.

Reference values (K-Light 125)