The spark plug is a small but critical part that ignites the fuel-air mixture inside the engine. When it wears or fouls, the engine is hard to start, misfires, does not pick up and burns more fuel. Since it is a cheap part, there is no point delaying its replacement; but simply swapping the plug is not always the solution, because the look of the plug also tells you what is going on inside the engine. In this guide we explain when to change it, how to tell it is worn out and how to "read" the plug.
At what mileage is a spark plug changed?
There is no fixed mileage; the replacement interval is written in the maintenance schedule in the engine's manual and varies with the plug type. For example, in the Turkish owner's manual for Honda's CB125F (SC74 type) the plug is inspected at 4,000 km and replaced at 8,000 km; that is, on this model a new plug is fitted every 8,000 km. The same manual gives NGK CPR7EA-9 as the standard plug and a 0.80–0.90 mm electrode gap.
On larger engines and with iridium-tipped plugs the interval may be longer. Whatever the motorcycle, go by the value in the manual and, even if the mileage has not come up, have the plug checked if you see the symptoms below. The full maintenance schedule is in the maintenance guide.
Symptoms that a spark plug is worn out
- Hard starting: If you have to crank the starter for a long time, especially on cold mornings.
- Misfiring: The engine running in fits and starts while accelerating or at a steady speed.
- Uneven idle: Revs hunting at idle, the engine vibrating or stalling. On the CB125F the idle speed is 1,500 ± 100 rpm; find out your motorcycle's normal value from the manual.
- Not picking up, loss of power: The engine responding late and reluctantly when you open the throttle.
- Increased fuel consumption: When the mixture does not burn fully, you cover the same distance on more fuel.
Remember that these symptoms do not come from the spark plug alone; fuel, the air filter and the battery can produce the same picture. We went through the possible causes in order in the misfiring and stalling guide.
How do you read a spark plug?
The colour and deposits on the tip of the plug you remove give clues about how the engine is burning. Make the assessment after a normal ride, not after a short spell of idling.
| Appearance | What it may mean |
|---|---|
| Light brown or grey, little deposit | Normal combustion. If the electrode is worn, though, it may still be time to replace it. |
| Dry, black, sooty (carbon fouling) | Rich mixture, constant low-speed and short-distance use, a plug with a heat range too cold for the engine, weakening in the ignition system |
| Wet, oily black deposit | Oil getting into the combustion chamber; a worn ring or cylinder. The likelihood rises if there is also blue smoke from the exhaust. |
| White, burnt-looking tip, blisters | Overheating; a plug with a heat range too hot for the engine, or a lean mixture. Show it to a workshop. |
According to NGK's technical information, the tip of the plug must reach about 450 °C to be able to burn off the deposits on it and clean itself. On a motorcycle constantly used for short distances at low speed, the tip cannot reach this temperature and carbon builds up. NGK lists constant low-speed and short-distance use among the causes of carbon fouling; on a motorcycle that only ever does a few kilometres in the city, the plug therefore needs looking at more often.
The picture becomes clearer when you assess it together with the colour of the exhaust smoke; details are in the exhaust smoke guide.
Clean it or replace it?
NGK does not recommend cleaning a fouled plug: although technically possible, a cleaned plug does not work like a new one, and the worn electrode edges can wear even more during cleaning. Replace a fouled or worn plug. But as NGK also stresses, replacing a fouled plug with a new one may only postpone the problem: if the mixture, oil burning or ignition problem continues, the new plug will soon end up in the same state. The real cause needs to be found.
How do you choose the right plug?
- The code in the manual: The manual gives the exact code of the standard plug for your engine. For the CB125F this code is NGK CPR7EA-9. Use the same code or its equivalent in the manufacturer's cross-reference table.
- Heat range: In NGK codes, a small number means a hotter plug and a large number a colder plug. Do not change the heat range on a whim; the wrong value leads either to fouling or to overheating.
- Electrode gap: It must match the value in the manual. Check the gap of the new plug with a feeler gauge before fitting it, too.
- Thread length and diameter: If the code is right, these are right too; a plug that looks similar but has a different thread length can damage the engine.
You can find plugs on the spark plugs page and the tools you need on the tool sets page.
The plug cap and lead matter too
Sometimes the plug is fine but the spark does not reach it. A cracked plug cap or one coated in oil and moisture, an oxidised connection or a worn lead can cause misfiring, especially in rainy weather and after the engine has been washed. When checking the plug, also look inside the cap and at the lead; the cap should seat fully and not wobble. When washing the engine, do not aim pressurised water at the plug area; for cleaning advice, see the engine washing guide.
If you are changing the plug yourself
- Let the engine cool; the thread in a hot cylinder head is easily damaged.
- Pull the plug cap off by the cap itself, not by the lead.
- Clean around the plug seat so that dust and sand do not fall into the cylinder.
- Remove the plug with a plug spanner and look at its tip.
- Check the electrode gap of the new plug.
- Screw the plug in by hand first until it turns freely; if it is hard going in the first turns, the thread may have crossed, so back it out.
- Do the final tightening to the torque value in the manual or the instructions on the plug's packaging; over-tightening damages the thread.
- Push the cap on until it seats with a click, then start the engine and listen to the idle.
In the CB125F manual, Honda marks spark plug work as "intermediate" level maintenance: if you do not have the right tools and mechanical skill, leave the job to a workshop. On some motorcycles the plug sits under the fuel tank or the fairing, and removing it takes more work.
Conclusion
The spark plug replacement interval is in the manual; on the Honda CB125F it is every 8,000 km. Hard starting, misfiring, uneven idle and rising fuel consumption show it is time to check the plug. The colour of the plug you remove tells you how the engine is burning: a black sooty tip points to a rich mixture or short-distance use, an oily tip to oil burning and a white tip to overheating. Do not clean a fouled plug, replace it; but if the same problem recurs, have the cause found.
Frequently asked questions
At what mileage is a motorcycle spark plug changed?
It varies with the model and plug type and is in the manual's maintenance schedule. For example, on the Honda CB125F the plug is changed every 8,000 km.
How can you tell a spark plug is faulty?
Hard starting, misfiring, uneven idle, not picking up and increased fuel consumption are the most common symptoms. The same symptoms can also come from the fuel or air system; removing and checking the plug is the quickest first step.
What happens if no current reaches the spark plug?
The engine will not run or keeps misfiring. The problem may be in the plug, plug cap, lead, coil or ignition system. Because a spark test carries a risk of electric shock, it is safer to leave this check to a workshop.
Can a spark plug be cleaned and refitted?
NGK does not recommend it: a cleaned plug does not work like a new one and its electrodes can wear even more. Replace the fouled plug and investigate the cause of the fouling.