| Specification | Value |
|---|---|
| Model Year | 2024 |
| Engine Type | 125cc 2-Stroke, Liquid-cooled |
| Transmission Oil | 0.70 L (Yamalube 4 10W-40) |
| Spark Plug | NGK BR9EG |
| Front Tire | 80/100-21 51M (14.5 psi) |
| Rear Tire | 100/90-19 57M (14.5 psi) |
The 2024 Yamaha YZ 125 (B4XJ) stands as a testament to Yamaha’s commitment to the two-stroke platform. Engineering Highlight: The YZ125 holds the distinction of being the longest-running production motocross model in history, and the B4XJ generation represents the most significant aerodynamic and ergonomic overhaul in over a decade, utilizing a functional ‘straight-shot’ intake design to maximize volumetric efficiency at high RPMs.
Technical Review: Performance and Handling
The B4XJ variant continues to dominate the small-bore class through its exceptional power-to-weight ratio. The 125cc liquid-cooled engine features the Yamaha Power Valve System (YPVS), providing a broad spread of power across the rev range. For 2024, the focus remains on the KYB® Speed Sensitive System (SSS) front forks, which are widely considered the gold standard in motocross suspension, offering unmatched damping characteristics and rider feedback.
The chassis is a lightweight aluminum semi-double-cradle frame, engineered specifically to balance rigidity and flex. This allows the YZ 125 to maintain stability in rough sections while remaining the most flickable bike on the track.
Maintenance Guide: Professional Standards
1. Lubrication System
The gearbox requires 0.70 Liters of oil. MotoSpecsLab recommends Yamalube 4 (10W-40) or a high-quality SAE 10W-40 meeting API SG+ and JASO MA standards. For competitive use, oil should be changed every 5 hours of operation to ensure clutch longevity and gear protection.
2. Ignition and Combustion
The NGK BR9EG is the specified spark plug. It is critical to inspect the plug for fouling, especially if running rich fuel-to-oil ratios. Ensure the gap is set to factory specifications to prevent high-RPM misfires.
3. Wheel and Tire Management
Traction is managed by an 80/100-21 front and a 100/90-19 rear tire. For optimal contact patch performance on intermediate terrain, maintain a pressure of 1.0 bar (14.5 psi) in both tires. Lowering pressure slightly (to 12-13 psi) may be beneficial in deep sand, but increases the risk of pinch flats.
Editor’s Verdict
The Yamaha YZ 125 (B4XJ) remains the ultimate tool for developing rider technique. Its mechanical simplicity, combined with sophisticated suspension and a high-revving heart, makes it a formidable weapon in the right hands. Adhering to the 0.70L oil capacity and precise tire pressures outlined above is essential for maintaining its competitive edge.
Frequently Asked Questions
Q: Does the 2024 YZ 125 require a battery?
A: No, the YZ 125 B4XJ utilizes a kickstart system and a simplified electrical circuit without a battery.
Q: What is the fuel-to-oil mix ratio?
A: While varying by oil brand, Yamaha typically recommends a 30:1 or 40:1 ratio using high-quality 2-stroke racing oil.