Yamaha YZ 125 (2017–2021): A Purebred Motocross Legend Tested

Introduction

The Yamaha YZ 125 has long been the gateway to motocross glory for riders craving the razor-sharp responsiveness of a two-stroke engine. The 2017–2021 iteration of this iconic machine stays true to its roots, offering a blend of lightweight agility, explosive power delivery, and race-ready engineering. Designed for motocross purists and aspiring racers, this bike isn’t just a tool—it’s a teacher. After a day of thrashing it on the track, it’s clear why this generation remains a benchmark in the 125cc class.

---

Engine Performance: The Screaming Two-Stroke Soul

At the heart of the YZ 125 lies its 124cc liquid-cooled two-stroke engine, a design refined over decades. The Mikuni TMX 38 carburetor ensures crisp throttle response, and the reed-valve induction system delivers a punchy mid-range that’s addictive. With a compression ratio of 10.7:1, this engine thrives on revs.

Power Delivery: The YZ 125’s powerband is a classic two-stroke affair—forgiving at low RPMs but explosive once the needle climbs past 7,000 RPM. The hit isn’t as abrupt as older models, thanks to Yamaha’s focus on rideability, but it still demands respect. Premix fuel (recommended at 30:1) is a ritual, and the scent of synthetic oil adds to the visceral experience.

Gearbox: The six-speed transmission is slick and precise. Third and fourth gears are where the magic happens on most tracks, though the close ratios let you chase the powerband aggressively. Clutch pull is light, and the wet multi-plate clutch handles abuse without fading—a must for hard-charging starts.

Sound & Vibe: There’s nothing quite like the scream of a 125cc two-stroke at full tilt. The YZ 125’s exhaust note is sharp and urgent, punctuated by the mechanical snarl of the piston slapping through its cycle. Vibration is minimal, a testament to Yamaha’s balancing work.

---

Chassis & Suspension: Precision Meets Durability

The YZ 125’s semi-double cradle aluminum frame is a masterpiece of rigidity and flex. Weighing just 94 kg (207 lbs) with fluids, it’s a featherweight in a class where every gram matters.

KYB Suspension:
- Front: The 48 mm KYB Speed-Sensitive System (SSS) inverted fork offers 300 mm (11.8 inches) of travel. It’s plush over small bumps yet firm enough to handle big jumps. Adjusting the clickers (rebound/compression) takes minutes, and the difference is immediately noticeable.
- Rear: The KYB shock delivers 315 mm (12.4 inches) of travel. Sag is easy to set (aim for 100–105 mm), and the linkage system keeps the rear planted even on choppy exits.

Ergonomics: The 975 mm (38.4-inch) seat height is approachable for shorter riders, but the narrow frame makes it easy to grip with your knees. The handlebars feel neutral, and the Dunlop MX52 tires (80/100-21 front, 100/90-19 rear) provide predictable grip in most conditions.

---

Handling: Dance Like a Butterfly, Sting Like a Bee

The YZ 125’s 1,443 mm (56.8-inch) wheelbase and 26-degree rake strike a perfect balance between stability and flickability. On tight S-curves, it changes direction like a trials bike. Over tabletops, the chassis stays composed mid-air.

Brakes: The single 245 mm hydraulic discs (front and rear) lack the bite of higher-end models but offer progressive feel. Upgrading to sintered pads (available at MOTOPARTS.store) sharpens stopping power without breaking the bank.

Track Performance: This bike rewards aggression. Charge into corners, brake late, and let the engine’s top-end pull you out. On hardpack, lower the tire pressure slightly from the stock 1.0 Bar (14.5 PSI) for extra grip.

---

Competition: How the YZ 125 Stacks Up

KTM 125 SX

KTM’s 125 SX is lighter (92 kg / 203 lbs) and packs a more aggressive powerband. However, the YZ 125’s KYB suspension is widely regarded as superior for rough tracks. KTM’s linkage-less rear suspension can feel harsh on square-edged bumps.

Husqvarna TC 125

The Husky shares KTM’s engine but adds a hydraulic clutch and composite subframe. While premium, it’s pricier to maintain. Yamaha’s reliability and cheaper part costs give the YZ an edge for budget-conscious racers.

Honda CRF125R

Honda’s four-stroke CRF125R is friendlier for beginners but lacks the YZ’s adrenaline-inducing power curve. For serious MX training, the two-stroke’s urgency is unmatched.

---

Maintenance: Keeping the YZ 125 in Fighting Shape

Critical Checks

1. Premix Ratio: Stick to 30:1 with a high-quality synthetic oil. A ratio ruler (available at MOTOPARTS.store) eliminates guesswork.
2. Spark Plugs: The NGK BR9EVX (standard) or BR9EIX (iridium) should be gapped to 0.6–0.7 mm. Carry spares—two-strokes are plug-fussy when jetting isn’t perfect.
3. Air Filter: Clean after every ride. Our Twin-Air filter kits include biodegradable cleaner for hassle-free maintenance.
4. Chain & Sprockets: The 520 chain lasts longer with regular lubrication. Swap the stock 13/48 sprockets for steel units if durability is a priority.

Upgrade Recommendations

• Exhaust: A DEP or FMF expansion pipe boosts mid-range torque.
• Suspension Tuning: Send your KYB components to a specialist for revalving.
• Handguards: Acerbis Rally Pro guards protect levers during spills.

---

Conclusion: The Two-Stroke Teacher

The Yamaha YZ 125 (2017–2021) isn’t just a bike—it’s a rite of passage. It punishes laziness and rewards precision, making you a better rider with every lap. While newer Euro bikes offer flashier tech, the YZ’s bulletproof reliability and timeless handling keep it relevant. Whether you’re chasing podiums or mastering clutch control, this machine delivers grins per gallon that four-strokes can’t match.

Ready to personalize your YZ 125? Explore MOTOPARTS.store’s curated selection of performance upgrades, maintenance kits, and rider gear tailored for motocross warriors.

---

Specifications sheet

---