Reading The Color of Spark Plug in 2-Stroke to Determine Jetting

Greetings, fellow motoring enthusiasts! Today, we will dive deeper into 2-stroke motorcycle tuning, focusing specifically on the ins and outs of reading spark plugs in a two-stroke engine. Although the focus is on two strokes, the principles we will discuss also apply to our four-stroke friends. The art of deciphering your spark plug messages can reveal much information about your engine’s performance, and we’re here to help you unravel the mystery.

Actually, this is a technique of reading the spark plug through color shared by a person who is very knowledgeable in the world of dirt bikes on his YouTube channel. I myself have not gone this far in terms of reading spark plug colors, but his explanation was so intriguing that I have to share it with you all. Now, where should we start?

Where and What to Look For?

First off, let’s address the dilemma that often plagues riders: where exactly should you read the spark plug, and what clues are hidden within? The spark plug serves as a window into your engine’s inner workings, providing insights into air-fuel ratios, ignition timing, heat ranges, and even the presence of detonation. When embarking on this diagnostic journey, ensure that you have a fresh set of spark plugs ready, as old ones won’t yield accurate results.

The Initial Test

To gain a general understanding of your bike’s overall performance, perform the initial test. Install a new spark plug, run the engine for about 30 minutes, and then extract the plug for inspection. This test will help reveal the air and fuel mixture, oil and fuel ratios, and even the ignition timing to a certain extent.

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Interpreting the Colors

analyze is what color at the top of the spark plug
on this circle determines the overall running

The first one to analyze is what color at the top of the spark plug ground. This hue can reveal the air-fuel ratio your engine is running on. However, it’s not going to tell you whether your main jet or pilot is too rich or too lean. It is about the overall jetting.

A tan color is generally ideal, indicating a balanced mixture. A black appearance hints at a rich mixture, while a lack of color suggests a lean condition. Keep in mind that this test offers a holistic view and might not pinpoint specific circuit issues.

Oil Deposits and Premix Ratios

spark plug oil and fuel mixture

For two strokes, the oil and fuel mixture is essential, and its presence on the spark plug can convey valuable information. If the thread and tip are adorned with the right amount of oil, your mixture might be on point. If there’s excessive oil, consider adjusting your premix ratio. For example, track use might necessitate a different ratio compared to trail riding.

Signs of Detonation

Signs of Detonation on spark plug
pre-ignition here

Detonation or pre-ignition can be detrimental to your engine’s health. Inspecting the spark plug can reveal signs of melting between the electrode and insulator, or black and silver specks at the insulator’s base. These signals indicate detonation. Fortunately, our guide explains how to perform a chop test to delve deeper into this matter.

Ignition Timing and Heat Range

Spark plug electrodes

Spark plug electrodes can provide an indication of ignition timing. A distance between the light-colored tips of more than 1/2 mm from the tip of the electrode can indicate whether the timing needs adjustment, retarding the ignition timing is recommended.

Furthermore, the spark plug’s tip itself can disclose the heat range. A burnt plating at the tip suggests a spark plug that’s too cold:

 spark plug that's too cold

while the same issue at the base implies a spark plug that’s too hot:

 spark plug that's too hot

It’s all about achieving that optimal balance.

spark plug burnt good
If it’s burnt here you’re good

The Chop Test: Unveiling Main Jet Sizing

The chop test is the ultimate diagnostic tool to assess the main jet size. Here, you replace the existing spark plug with a new one, run the bike through the gears, shut it off, and carefully disassemble the spark plug to examine the insulator. A two-millimeter thick brown or tan ring indicates a balanced main jet size. A thicker or black ring might suggest richness, while a thinner or whitish ring could signify a lean condition.

chop the spark plug. cut it


Reading a spark plug is more than just a mechanical ritual; it’s an art form that can reveal your bike’s innermost secrets. Understanding the colors, deposits, and patterns on your spark plug can equip you with the knowledge to fine-tune your engine’s performance. Check out his video below:

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