The Spark Plug - The Source of Ignition

Motorcycle engines need a source of ignition to run and that's the job of the spark plug. Spark plugs are produced with 8, 10, 12, 14 and 18mm diameter threads; the thread reach can also vary in length. Modern motorcycle engine designs often place severe constraints on spark plug design and positioning within the combustion chamber. The available space for locating the spark plug is ever decreasing due to increased valve sizes, larger coolant passageways and more recently, fuel injectors.

If you want to replace your spark plug make sure there is no debris around the plug prior to removal. Before inserting a new plug, make sure the threads in the cylinder head are clean and in good condition. Install the spark plug by hand until it seats – a length of rubber tubing pushed over the insulator can be a useful aid where access is difficult. Tighten referring to the instructions on the spark plug box – which can differ between make and model due to individual spark plug design (e.g. seating type, thread diameter and gasket material).

Gold palladium, iredium or copper; which makes the best electrode?

The materials used at the center electrode tip are nickel alloy, gold palladium, platinum or iridium. The melting point of copper is too low to allow its use on the electrode tips; however it is deeply inserted into the center electrode core of most spark plugs providing improved thermal conductivity and an ultra wide heat range. The smaller diameter center electrode designs offer several advantages including a reduction in required voltage, more consistent spark position, reduction in 'quench effect', more complete combustion and lower emissions.
As materials vary in their resistance to erosion, there has to be a balance between performance and spark plug life. Least durable is nickel alloy, the diameter is typically between 1.6 to 2.9mm (depending on make and model) to ensure a good service life. The hardest material is the precious metal iridium, which makes it the ideal choice for performance and longer life spark plugs. The iridium alloy fine wire which is laser welded to the tip of the center electrode allows a reduction in electrode diameter down to 0.4mm.

A spark plug in a four-stroke engine capable of revving to 10,000rpm will produce 166 sparks per second if a 'wasted spark' ignition system is used. A spark plug in a more highly tuned or competition engine would produce in excess of this depending on maximum rpm and ignition system utilized.

Is there any advantage behind a split-fed ground electrode?

There have been many different spark plug designs produced by various spark plug manufacturers over a great many years in a quest for improved performance. Although there are specific requirements for some engines to have special ground electrode designs, usually for increased service life or strength, no matter how many discharge points a plug has, simultaneous multiple sparks are not possible – only one spark will be produced per discharge event.

Can spark plugs really read your motorcycle's fueling?

The color of the insulator around the center electrode on the firing end of a spark plus is a good indication of the condition of the engine/fueling set-up. The ideal color is often described as 'biscuit brown'. In relation to fueling, an insulator that is very white often indicates a weak fuel mixture and that a plug is getting too hot. An insulator that is sooty/black indicates that the plug is having difficulty coping with rich fuel conditions.
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