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How to inspect and repair/replace spark plug in scooter.

How to inspect, repair scooter spark plug. When to replace, how to replace and other spark plug related trouble shooting.


Spark plugs usually won’t get faulty often. Sometimes a spark plug can continue working as long as 20 km and more.

If a spark plug is getting frequently ruined, reason is that there is another part that is faulty.

If your scooter is working fine after replacing spark plug. But issues happen again after few rides, that means there is a primary problem that need to be solved.

For example, symptoms like very lean spark plug, or fuel on spark plug etc. are secondary issues that happens because of other faulty parts.

Testing whether spark plug is good or not.

How to do spark plug test — remove spark plug from engine head and attach to the end of magnetic coil wire. Place the spark plug tip near any grounded part of the vehicle and crank the engine.

Points to note — color of spark. — color of the spark plug is a direct indicator of many things like, any primary faults. If it is a bright blue, it is a good sign.

spark plug correct color bright blue

A faulty spark plug will produce a color other than bright blue, may be orange/yellow. If it is not bright blue or the spark is very weak, there is some issue.

Color of spark - bright white spark. Bright blue spark.

Spark travel route. — you can inspect the travel route of the spark. Spark traveling straight from one tip to the other or sideways is a good indication.

Corrosion & carbon residue. — unusual corrosion , too much carbon residue, spark plug tip losing its shape & eroded etc. all are indication that it is time to change the spark plug.


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How to clean & fix spark-plug.

If the spark plug is too rich, or too lean, you should first fix the air fuel ratio, by tuning the carburetor, fixing any vacuum leak or similar issue.

After the primary issue is fixed, inspect the spark plug and if it is in good shape, clean it with a (small)wire brush(or a blade) and adjust the gap according to manufacturer’s recommendation.

What does color of spark plug says about air fuel mixture.

Spark plug analysis

Too lean spark plug — no presence of oil, fuel, or carbon.

Too rich spark plug — This will look really dark, or oily, or mixed with fuel.

Perfect spark plug — if both air and fuel is mixed in perfect ratio, you will see a gray spark plug. Not too much carbon deposit or not too lean.

Spark plug gap.

You can use a gauge to measure the gap and then reduce or increase the gap based on measurement. The recommended spark plug gap for each manufacturer is different, and this info is available in the user manual.

Spark plug gap can be adjusted using a hard, thin object. Just press slightly on top horizontally to reduce gap size.

You can use the same thin coin type material(gauge disc or stick, preferred) to increase the size of gap also.

Written on December 3, 2021
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