Replacing the spark plugs is part of a normal motorcycle service, but why do they need replacing and why does the bike feel sharper and more responsive after the plugs have been replaced?
The pictures below show a new and used motorcycle spark plugs side by side.
New and used motorcycle spark plugs
Close up of the spark plug heads showing the electrodes
The spark plugs above are both the same make and model, the plug on the right is brand new and unused, the plug on the left had been used for approx 12,000 miles.
As spark plugs wear, the spark they generate loses it’s power (a common fault with bikes not starting is attributed to a ‘weak spark’). If you look at the top of the spark plug, just underneath the curved metal top (known as the ground or side electrode), you’ll see a small, almost black part known as the electrode.
As the spark plug wears with mileage and use, the electrode wears and develops a slightly curved edge, as opposed to a new plug, that has a sharp, 90 degree edge.
Also, as the plug wears the electrode wears down which increases the gap where the spark is generated causing the spark to weaken.
New and used spark plug electrodes showing the sharp edges of a new plug and the worn edges of a used plug electrode
Old and new spark plug electrodes side by side
Apart from the dirt and carbon build up (which can reduce the effectiveness of a spark plug), the wear on the electrode greatly reduces the power of the electrical spark generated by the spark plug.
New spark plugs will have a crisp, strong white or slightly blue spark, as the plug wears the spark becomes less bright (weaker) and is more yellow in colour.
This is the main reason motorbikes and scooters can seem alot more responsive when the spark plugs are changed.
It’s worth bearing in mind that other factors can effect how a spark plug operates:
- Mileage (the higher the mileage the more ‘wear’ there’s likely to be on the plug)
- Water ingress (through the air filter system) can effect a spark plugs spark.
- Poor fuel. Old petrol or fuel that’s contaminated with water can drastically reduce the spark performance.
- Faulty plug coils or spark plug caps.
The used plug above had only done a few thousand miles and is already showing signs of wear to the electrode.
Spark plugs are usually the 1st thing that we (and most garages) will recommend being replaced if there’s running problems with a motorcycle engine, this is because they can often by the source of the problem and they are cheap to replace (better to replace them then it’s one part you can rule out if there’s still problems and it’s cheaper to replace plugs then expensive parts like coils etc).
Disclaimers:
The information provided on this page is ‘to the best of our knowledge’ and should not be taken as 100% accurate!!
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