If your Yamaha YZF-R125 is struggling to start or misfiring, the spark plug is the first place to look. While a visual check is helpful, a multimeter test can reveal internal failures that aren't visible to the eye.
Preparation
Remove the spark plug and clean the electrode tip. Carbon buildup can cause erratic readings, so ensure you have a clean contact point for your multimeter probes.
1. Internal Resistance Test
This test ensures the internal conductor of the spark plug is intact.
The Process: Set your multimeter to the Ohms (Ω) setting. Place one probe on the top terminal (where the HT lead cap connects) and the other probe on the center electrode tip.
The Result: You should see a very low reading (e.g., around 1.2 ohms). If the reading is extremely high or fluctuates wildly, the internal conductor is failing.
2. Earth Leakage Test
This test checks if the spark is "leaking" through the ceramic insulator to the metal body instead of jumping the gap at the tip.
The Process: Keep the meter on the Ohms setting. Place one probe on the top terminal and the other on the metal threaded body of the plug.
The Result: Your meter should read "OL" (Open Loop). This indicates there is no electrical path between the terminal and the body, meaning the insulator is doing its job. If you get any numerical reading, the plug is faulty and the spark will likely short out before reaching the combustion chamber.
When to Replace
If your spark plug fails either of these tests, replace it immediately. For the YZF-R125, it is recommended to use the standard NGK plug specified in your manual (usually NGK CR8E or CR9E depending on your model year). Always check the "gap" between the electrodes with a feeler gauge before installation.
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