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Self motorcycle maintenance – Spark plugs

No fire no power

Spark plugs as the name suggest generates a spark to ignite the air-fuel mixture inside the cylinder. It’s a matchbox of the engine just a little complicated.

Spark plugs works under heavy voltage like in the range of 25k to 45k volts to create a connection in the air gap between its central and ground electrode.

In case you might be wondering where does this much voltage come up to the spark plug since the battery usually provides 12 volts and alternator to0 provides voltage in the range of 12 volts to 20 volts.

Here is the answer, HT (high tension) coil sitting between ignition systems like CDI and TCI, steps up the voltage to thousands of volts. It is then passed on to spark plugs when the piston reaches TDC (top dead centre) of the power stroke (3 picture in below image).

4 stroke motorcycle diagram

Source: https://www.vectorstock.com/royalty-free-vector/four-stroke-engine-vector-20120974


Since the electrode ends face the inside of the combustion chamber it has to bear heavy compression of the cylinder. Though these plugs are manufactured to lasts longs however in cases of improper combustion, the wear and tear of the plug are increased thereby decreasing the performance of the engine.

When to change the spark plugs

When servicing the motorcycle, all the spark plugs should be removed and examined for the gap between the metals and colour on the surface.

Heres the chart to correlate the condition of spark plugs.

spark plug maintenance chart

Spark plug colour comparison chart

In case the plugs are really in bad shape, replacing them is a better option.

In case the gap has reduced or increased between the iron rods, it can be fixed by increasing or decreasing the distance using simple tools like T screwdriver. But this fix expects that you are aware of the manufacturer recommended distance between the two electrodes.

For example, this is the specification provided in Bajaj Dominar user manual regarding the spark plug gap.

If the gap between the electrodes is not correct, there may be issues like reduced time of sparks, reduced density of sparks. Such issues affect the correct burning of fuel mixture and as a result, enough force is not produced to push the piston down optimally, thereby reducing the torque.

When replacing the spark plugs, always go with manufacturers recommended spark plugs. Champion, NGK, Bosch are some known brands.

Couple of videos I recommend to watch regarding spark plugs


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Hello, I am Niraj. I define myself as an amateur photographer, biker and seeker. I like to connect with like-minded friends and share experiences and stories from the place, people and time. I believe life is an endless journey and our actions no matter how small affects this infinite universe in some way or another. So let's not stop and keep our work going.

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