Yamaha YBR125 Missing Issue Fix
What causes it ?
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The Yamaha YBR125 is notoriously known for experiencing missing or bogging issues. Here is a list of reasons that can contribute to this problem.
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Spark plug Gap: Adjust the spark plug gap to between 0.60mm and 0.70mm, and check if this resolves the issue.
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Wrong Petrol Octane: Using low-quality fuel or incorrect petrol octane can lead to engine missing. Opt for 92-95 octane fuel to ensure optimal performance.
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Cause 1:
The Yamaha YBR125 is equipped with a Constant Velocity (CV) carburetor, which operates by lifting the needle jet in response to air velocity. If the air filter becomes old or clogged, it leads to a drop in air pressure within the venture, causing the engine to bog or miss.
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Replace the air filter every 4-6 months or every 12,000 km, whichever comes first, to help prevent issues with engine missing. This simple maintenance step ensures your engine breathes clean air and runs smoothly.
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Cause 2:
The Yamaha YBR125 features three different types of fuel filters: the first is located on the petcock, the second underneath the petcock, and the third inside the carburetor, behind the float jet. This triple-filter system ensures optimal fuel cleanliness and engine performance.
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Fix : Clean all three fuel filters and replace the paper fuel filter to effectively fix the missing problem. This routine maintenance ensures your engine receives clean, uninterrupted fuel flow for optimal performance.
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Issue 2 : Another issue with the Yamaha YBR125 is that it can be difficult to start in winter.
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Fix : In the summer, set the carburetor to 2 and a half turns and adjust the RPM speed to between 1300-1500 RPM. In winter, turn the carburetor out to 3 turns and maintain the idle speed at 1300-1500 RPM. These adjustments help ensure smooth operation of your Yamaha YBR125 throughout the seasons.
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Fine-tune your motorcycle by setting the idle RPM between 1300-1500 and adjusting the idle mixture screw 2 and a half turns from the closed position. Start the motorcycle with the choke on, leave it for 10 seconds, then close it. Once the engine warms up, turn the idle mixture screw to the right, a quarter turn at a time, stopping when the engine RPM begins to decrease. From this point, turn the screw to the left, counting the turns until the RPM becomes irregular. Divide this number by the value where the RPM first started to decrease. Finally, adjust the screw to the right by this calculated value to optimize the engine's performance.
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For example: If the RPM begins to drop at 2 turns to the right, and becomes irregular at 3 turns to the left, then the correct mixture setting is 2 and a half turns. This is referred to as balancing the mixture. Adjust the mixture to find a balance between slightly low and high, and irregular RPMs to achieve optimal results. Set the idle RPM between 1300-1500 RPM. At this point, you should notice a smoother idling engine.
Issue 3 : In the summer, if the motorcycle starts to sputter or struggles to rev when you open the throttle, it may indicate an issue that needs attention.
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Fix : In summer, adjust the needle jet to the third position from the top to achieve an optimal air-fuel mixture. In winter, move it to the fourth position for best performance in colder conditions. This adjustment ensures your motorcycle runs smoothly throughout the seasons.
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Cause 3 : An electrical short can also cause issues with engine missing, disrupting your ride.
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Fix : Open the speedometer and light cover to check for any exposed wires. Ensure all wires are properly covered to prevent short circuits and keep your motorcycle running smoothly.
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Cause 4: If you adjust or change the spark plug gap, ensure the spark plug is tightened properly. A loose spark plug can lead to engine missing, affecting your motorcycle's performance.
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Fix : When tightening an old spark plug, turn it 45 to 90 degrees to the right. For a new spark plug, first tighten it 180 degrees, then loosen it, and tighten it again to 45 to 90 degrees. This method ensures a secure and effective fit, optimizing your motorcycle's engine performance.
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Cause 5: If your engine misses at wide open throttle, allow it to fully warm up before taking further action. If the issue persists, consider rejetting the carburetor's main jet. When testing the main jet, use the exact type as the stock jet. Ensure the engine is fully warmed up before reading the spark plug, and use a new spark plug for testing, as old ones can be difficult to read accurately. This approach ensures you get precise results and maintain optimal engine performance.
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Note: You will need to rejet the main jet whenever the weather shifts from winter to summer, or vice versa. Failing to do so risks the motorcycle running lean, which can cause engine damage or reduce its lifespan. This precaution ensures your engine performs optimally across different seasons.
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Rejetting Tips: If your motorcycle hesitates or loses power at wide open throttle, consider using a main jet that is two sizes larger, and continue increasing the size until you achieve maximum RPM without power loss. However, if the motorcycle sputters or repeatedly loses and regains power at wide open throttle until you reduce the throttle to 75%, this indicates that the main jet is too large. In such cases, reduce the main jet size by two increments until you attain a consistent power output. This fine-tuning ensures your ride is smooth and responsive across all throttle ranges.
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Rejetting requires patience—ensure the engine is fully warmed up before opening the throttle wide. It's crucial for the engine to reach operating temperature before taking any performance readings, as this ensures accurate adjustments and optimal engine function.
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Note: Not all jets are the same; sometimes the number on them measures drill size, and sometimes they measure fuel flow. Try to find a jet kit that includes at least 30 different jet sizes around your motorcycle's stock jet size. This variety will make it easier to precisely tune your motorcycle's main jet for optimal performance.
Learn carburetor tuning in following articles.