Today I changed the spark plug of my Honda CBR125R. The installation of a new spark plug was very simple, it took just 15 minutes. First of all, you need to remove the fairings on the right side of the motorcycle. There are just 3 screws that you need to remove, please refer to the Honda CBR125R Service Manual for additional info.
Then, it is necessary to remove the black cap on the spark plug. When you do it by yourself, be careful to apply the correct force and to not destroy the cap.
You need a particular tool, in order to remove the spark plug. In my case, this tool was included in a "motorcycle maintenance kit" which I bought in a home center for about 1,500 Yen (10 Euro).
Using this tool, I removed the old spark plug, which was a NGK CR8E: it is the official spark plug for the Honda CBR125R. It is very cheap (about 600 Yen, 4 Euro), but I decided to substitute it with an iridium spark plug. When I removed the old spark plug, the electrode looked like below: the electrode color was white/brown. I am not an expert of judging by the color, but I would say that maybe the air-fuel mixture is a bit lean (too much air). Maybe I should increase the injection time a bit.
After that, I mounted the new spark plug. I chose an NGK CR9EIX (iridium type), which I bought on Amazon Japan for about 1000 Yen (7 Euro). The picture below clearly shows that my cylinder is the Athena 166cc bore-up kit, and not the original Honda's 125cc cylinder.
Each spark plug has a "heat range", which expresses the attitude of the plug to dissipate the heat. It should be chosen according to the engine type and usage of the motorcycle. In my case, since I have a bore-up kit, and I like to open the gas a bit when the signal turns from "red" to "green", I suppose that the engine has to dissipate more heat than a an average CBR125R rider: for this reason, I decided to mount a spark plug with heat range 9, instead of the original 8. According to the NGK website:
Generally, low power engines such as lawn mowers don’t produce a large amount of heat, hence use a low heat range (or hot) spark plug such as a 4 heat range. This means the spark plug will heat up easily and reach its optimal operating temperature. High performance engines on the other hand produce a large amount of heat, hence a high heat range (or cold) spark plug such as a 10 heat range needs to be used to resist the heat developed by the engine.
When the heat rating is too high:
The spark plug temperature remains too low and causes deposits to build up on the firing end; the deposits offer an electrical leakage path that gives rise to loss of sparks.
When the heat rating is too low:
The spark plug temperature rises too high and induces abnormal combustion (pre-ignition): this leads to melting of the spark plug electrodes as well as piston seizure and erosion.
Also, even if a bit more expensive (1.5-2 times higher) than a usual spark plug, i decided to buy an Iridium one. Since late in the 20th century Iridium has been used in alloy form as material of choice for construction of sparks plugs. Pure Iridium is difficult to work with, very brittle and does not have great electrical conductivity hence alloys are used to give the best combination of these properties. An Iridium alloy electrode is able to deliver good conductivity and extreme hardness. The result is that an Iridium electrode can be manufactured with an extremely small diameter, when compared to a traditional copper or even a platinum tipped electrode, without compromising the useful life of the electrode or spark plug. The narrow diameter concentrates the electrical charge and therefore a much lower voltage is required to jump the spark gap. This is beneficial in several ways:
- A lower voltage is required, this reduces load on a vehicle's ignition system (less strain = less chance of failure)
- A larger gap can be supported without increasing load on the ignition system (larger gap = larger spark = more efficient combustion)
- A more precise spark every time, delivered in the correct position and with the correct timing (more efficient combustion)