Yamaha YZ 125 (2005-2016): A Motocross Legend Revisited

For over a decade, the Yamaha YZ 125 stood as a benchmark in the 125cc two-stroke motocross segment. Between 2005 and 2016, this bike earned a cult following for its razor-sharp agility, explosive power delivery, and race-ready DNA. As a lightweight weapon designed for riders who thrive on momentum and precision, the YZ 125 remains a favorite among motocross purists. Let’s dissect what makes this generation iconic—and why it still turns heads on tracks today.

---

The Heartbeat: A Feral Two-Stroke Engine

At the core of the YZ 125 lies its 124cc liquid-cooled two-stroke engine. With a bore and stroke of 54.0 x 54.5 mm, this powerplant is all about high-revving excitement. The Mikuni TMX 38mm carburetor (a constant throughout this generation) delivers crisp throttle response, though it demands meticulous jetting for optimal performance in varying climates.

Power Delivery:
The YZ 125’s engine isn’t for the faint-hearted. It’s peaky, rewarding aggressive riders who keep the revs dancing near the 11,000 RPM redline. The hit comes hard around 8,000 RPM, propelling the bike forward with a ferocious bark that’s music to any two-stroke lover’s ears. With 39 PS (29 kW) on tap, it’s a reminder that raw power-to-weight ratio still rules in motocross.

Gearbox:
Early models (2005-2006) used a 5-speed transmission, but Yamaha upgraded to a 6-speed box in 2007—a welcome change for tighter tracks. The clutch (a wet multi-plate design) requires a firm pull but offers excellent modulation for controlling power slides or lofting the front wheel over obstacles.

The Two-Stroke Ritual:
Ownership means embracing the premix life. The YZ 125’s lubrication system demands a fuel-to-oil ratio of 30:1 to 32:1, depending on riding style. Forget fuel injection or oil reservoirs; this is analog motocross at its purest.

---

Chassis and Suspension: Precision Meets Durability

Yamaha’s chassis philosophy for the YZ 125 centered on minimalist weight and maximal feedback. The steel frame (2005-2009) evolved to an aluminum perimeter frame in 2010, shedding weight and sharpening cornering precision. At 86–94 kg (190–207 lbs) ready-to-ride, the YZ 125 feels like an extension of your body mid-air or through rutted turns.

Suspension Setup:
- Front: The 48mm Kayaba inverted forks (2005-2009) transitioned to speed-sensitive system inverted forks post-2010, offering 300 mm (11.8 inches) of travel. Adjustability is key here—compression and rebound damping can be tuned for everything from hardpack to sandy whoops.
- Rear: A fully adjustable Kayaba monoshock handles 315 mm (12.4 inches) of travel. Later models added separate high/low-speed compression adjusters, a boon for fine-tuning rear-end squat under acceleration.

Ergonomics:
With a seat height of 998 mm (39.3 inches), the YZ 125 caters to taller riders, though its narrow aluminum frame makes it manageable for shorter pilots. The rider triangle—pegs, seat, bars—encourages an aggressive attack position, ideal for charging through berms or scrubbing jumps.

---

Handling: Where the YZ 125 Shines

Throw the YZ 125 into a corner, and it responds like a scalpel. The lightweight chassis (wheelbase: 1,443 mm / 56.8 inches) and 80/100-21 front tire provide telepathic front-end grip. On exit, the 100/90-19 rear tire hooks up predictably, though hard-pack riders often swap to a softer compound for extra bite.

Brakes:
Single 245mm discs front and rear deliver adequate stopping power for a 125cc machine. While not as aggressive as European competitors, the Nissin calipers offer progressive feel—critical for modulating speed before jumps or tight corners.

Track Manners:
This bike thrives on momentum. Without the torque of a four-stroke, you’ll live on the clutch to keep the engine boiling. But master the art of carrying speed, and the YZ 125 flatters with its ability to pivot, carve, and correct mid-air.

---

Competition: How the YZ 125 Stacks Up

KTM 125 SX

KTM’s 125 SX packs a harder-hitting engine and hydraulic clutch, but its suspension (WP AER forks) can feel harsh on choppy tracks. The YZ 125 counters with smoother damping and a more forgiving chassis.

Honda CR125R

Honda’s offering (discontinued in 2007) featured a case-reed engine with stronger midrange, but its steel frame felt heavier in transitions. Yamaha’s aluminum frame post-2010 gives it the edge in flickability.

Suzuki RM125

The RM125 rivaled the YZ 125 in cornering prowess thanks to its Showa suspension, but reliability issues and dated ergonomics left it trailing Yamaha’s consistency.

Verdict: The YZ 125 isn’t the most powerful, but its balanced package—durable engine, plush suspension, and agile chassis—makes it the most versatile choice for riders honing their skills.

---

Maintenance: Keeping the YZ 125 in Fighting Shape

1. Premix Faithfully: Stick to 30:1 or 32:1 ratios with high-quality synthetic oil. A lean mix risks seizures; too much oil fouls plugs.
2. Air Filter Care: Clean the foam filter after every ride. Consider a twin-air filter upgrade for better airflow and debris protection.
3. Suspension Service: Rebuild forks and shock every 40–50 hours. Upgrade to SKF seals for durability in muddy conditions.
4. Spark Plugs: NGK BR9EVX or BR9EIX plugs are essential. Gap them to 0.6–0.7 mm for consistent ignition.
5. Chain & Sprockets: The stock 13/48 gearing works for most tracks. Replace the chain at 0.5% elongation—a DID ERT2 chain adds longevity.

Pro Tip: Flush the cooling system annually with distilled water and Honda Type 2 coolant to prevent corrosion in the liquid-cooled engine.

---

Conclusion: A Timeless Motocross Icon

The Yamaha YZ 125 (2005–2016) isn’t just a bike—it’s a rite of passage. Its two-stroke scream teaches clutch control, throttle discipline, and the art of momentum. While modern four-strokes dominate podiums, the YZ 125 remains a testament to the raw, unfiltered joy of motocross. Whether you’re resurrecting a used gem or fine-tuning a race-ready machine, MOTOPARTS.store has the upgrades and OEM replacements to keep your YZ 125 alive and ripping.

Rev hard, shift harder.

---

Specifications sheet

---