🏷️ Basic Information

Make:

Yamaha

Model:

YZ125

Year:

2001

Vehicle Type:

Motocross Motorcycle

Model Generation:

Fifth Generation (1996-2001)

Chassis Code:

4SS1

👋 Introduction

Segment:

Competitive 125cc Motocross Segment

Target User:

Motocross racers and serious off-road enthusiasts looking for in-depth technical information for maintaining and repairing their Yamaha YZ125.

Competitors:

Honda CR125R, Kawasaki KX125, Suzuki RM125, KTM 125 SX.

Documentation Source:

Factory original service manual, ensuring accuracy and completeness.

Content Value:

Provides the same detailed information used by Yamaha technicians, enabling owners to perform professional-level maintenance and repairs.

📋 Manual Details

Primary Focus:

Repair and Maintenance

Scope:

Complete tear down and rebuild

Total Pages:

251

Type:

Factory Service Manual / Repair Manual

Language:

English

Format:

Digital Download (PDF)

📖 Manual Coverage

Engine:

Complete disassembly, inspection, and reassembly of the two-stroke engine, including top-end (cylinder, piston, rings, cylinder head), bottom-end (crankshaft, connecting rod, bearings, seals), cooling system, and lubrication system.

Transmission:

Service and repair of the gearbox, clutch assembly (including plates, springs, and actuation), shift drum, and shifting mechanisms.

Chassis:

Suspension (front forks, rear shock absorber, linkages), steering system, frame, subframe, footpegs, and all related components.

Fuel System:

Carburetor (Mikuni VM26SS) tuning, cleaning, and overhaul, fuel tank, fuel lines, and petcock.

Electrical System:

Ignition system (CDI, spark plug), lighting (if applicable for specific regional variants, though typically minimal on motocross bikes), wiring diagrams, troubleshooting of electrical faults, and component testing.

Braking System:

Front and rear hydraulic disc brake systems, including caliper overhaul, master cylinder service, brake line replacement, and fluid bleeding.

Exhaust System:

Removal, inspection, and reinstallation of the exhaust pipe and silencer.

Cooling System:

Radiator service, coolant replacement, hose inspection and replacement (for liquid-cooled models).

Maintenance:

Regular maintenance checks, oil changes, filter cleaning/replacement, chain adjustment, spark plug replacement, and pre-ride inspections.

Troubleshooting:

Diagnostic procedures for common issues, symptom-based troubleshooting guides, and diagnostic flowcharts.

Repair:

Step-by-step repair instructions for virtually all components, from minor adjustments to full engine rebuilds.

Diagnostics:

Detailed procedures for diagnosing performance issues, electrical faults, and mechanical failures.

Torque Specifications:

Exact torque values for all critical fasteners and components.

Depth:

Comprehensive, suitable for professional mechanics and experienced DIY enthusiasts. Covers from basic upkeep to full engine and chassis overhauls.

Visuals:

Includes detailed diagrams, photographs, and illustrations to guide procedures.

Diagnostic Tools:

Guidance on using common tools and techniques for diagnosis and repair.

📊 Manual Specifications

Format:

PDF

Searchability:

Text-searchable PDF for quick information retrieval.

Resolution:

High-resolution digital scans for clear text and images.

Binding Type:

N/A (Digital Download)

Paper Type:

N/A (Digital Download)

⚙️ Engine Specifications

Engine Type:

Liquid-cooled, 2-stroke, reed valve induction

Displacement:

124 cc

Bore X Stroke:

54 mm x 54.5 mm

Compression Ratio:

8.6:1 (low speed) / 9.0:1 (high speed) - Note: Yamaha often specified two ratios for two-stroke powerband characteristics.

Cylinder Layout:

Single cylinder

Carburetor:

Mikuni VM26SS, single

Horsepower:

Approximately 35-38 HP (estimated, varies with tuning and conditions)

Torque:

Approximately 17-19 lb-ft (estimated)

Fuel Type:

Unleaded gasoline (91 octane or higher recommended)

Fuel Mixture:

2-stroke oil/gasoline mix (refer to manual for precise ratio, typically 32:1 or 40:1)

Fuel Tank Capacity:

7.2 Liters (1.9 US gal)

🔧 Transmission

Transmission Type:

6-speed constant mesh

Clutch Type:

Wet, multi-plate

Gear Ratios:

Specific ratios are detailed within the manual (e.g., 1st: 2.xxx, 2nd: 1.xxx, etc.)

Final Drive:

Chain drive

Transmission Oil Type:

Yamaha Genuine Gear Oil (or equivalent API GL-4, SAE 80W)

📐 Dimensions

Length:

2,135 mm (84.1 in)

Width:

800 mm (31.5 in)

Height:

1,275 mm (50.2 in)

Wheelbase:

1,450 mm (57.1 in)

Ground Clearance:

335 mm (13.2 in)

Seat Height:

955 mm (37.6 in)

Dry Weight:

94 kg (207 lbs)

Wet Weight:

101 kg (223 lbs) (approximate, with all fluids)

Front Tire Size:

80/100-21 51M

Rear Tire Size:

100/90-19 57M

Wheel Bolt Pattern:

N/A (Spoked wheels with captive nuts)

🛡️ Safety Features

Crash Test Results:

Not Applicable (Motocross vehicles are not subject to standard road safety regulations and testing)

Braking System:

Hydraulic disc brakes front and rear for reliable stopping power.

Suspension:

Long-travel, adjustable suspension (Kayaba USD forks, Monoshock rear) for terrain absorption and control.

Frame Construction:

Lightweight yet strong aluminum frame for rider confidence and control.

Ergonomics:

Designed for optimal rider positioning and control during aggressive riding.

💡 Technology Features

Infotainment System:

N/A (Motocross motorcycle, no infotainment system)

Driver Assistance Features:

N/A (Motocross motorcycle, focus is on rider control and mechanical systems)

Connectivity Features:

N/A (Motocross motorcycle)

🔧 Maintenance

Oil Change Interval:

Frequent checks and changes recommended, especially under racing conditions. Refer to manual for specifics based on usage.

Air Filter Cleaning:

Recommended after every few rides or as needed based on dust conditions.

Spark Plug Replacement:

Regular checks and replacement based on condition and riding hours. Manual specifies intervals.

Top End Rebuild:

Recommended based on hours of use (e.g., 20-50 hours for racing, potentially longer for recreational use). Manual provides detailed inspection criteria.

Engine Oil Type:

High-quality 2-stroke engine oil, specifically formulated for motocross or racing applications. Yamaha recommends their own branded oils or equivalent JASO FD/FC certified products.

Coolant Type:

Permanent type antifreeze coolant, 50% solution with de-mineralized water. Yamaha recommends their Yamalube coolant or equivalent.

Brake Fluid Type:

DOT 4 hydraulic brake fluid.

Common Wear Items:

Clutch plates, piston rings, spark plugs, wheel bearings, swingarm bearings, and suspension seals are common wear items due to the nature of motocross riding.

Carburetor Tuning:

Proper carburetor jetting and tuning are crucial for optimal performance and can require adjustments based on altitude, temperature, and modifications. The manual provides guidance.

Engine Vibrations:

As with many high-performance 2-strokes, managing vibrations and ensuring all fasteners are secure is important.

📚 Vehicle History

Manufacturer:

Yamaha Motor Corporation, Japan

Motorcycle Legacy:

Yamaha has a long-standing and successful history in motocross, with numerous championships and innovations dating back to the 1970s. The YZ line is a cornerstone of their off-road performance reputation.

Yz125 Development:

The YZ125 has undergone continuous development, evolving from air-cooled beginnings to the liquid-cooled, reed-valve, powerful 2-stroke machines of the late 90s and early 2000s. The 2001 model represented a mature and highly competitive iteration of this lineage.

Key Advancements Around 2001:

Significant improvements in chassis design (aluminum frames), suspension technology, engine power delivery, and braking systems characterized this era of the YZ125.

Manufacturing Location:

Typically manufactured in Japan or through Yamaha's global production facilities.

🏆 Awards & Recognition

Motocross Popularity:

The YZ125 has consistently been a popular choice in its class for decades, earning a reputation for performance and reliability among amateur and professional racers.

Industry Reviews 2001:

While specific awards for the 2001 model year are not universally cataloged, reviews from the period typically praised its strong engine, agile handling, and competitive performance against its peers.