Segment:
High-performance 2-stroke motocross competition bike.
Competitors 2004:
Honda CR250R, Suzuki RM250, Kawasaki KX250.
Performance Focus:
Renowned for its powerful 2-stroke engine, agile handling, and competitive edge in motocross racing.
Design Philosophy:
Lightweight, potent, and rider-focused design for optimal performance on the track.
Engine Systems:
Top-end overhaul, piston, rings, connecting rod, crankshaft, bearings, cylinder, cylinder head, valve timing (if applicable to 2-stroke design), lubrication, cooling (air-cooled), ignition system, fuel delivery system (carburetor).
Drivetrain Systems:
Clutch (operation, adjustment, replacement), gearbox (disassembly, inspection, reassembly, gear ratios), drive chain and sprockets (maintenance, replacement), final drive.
Suspension Systems:
Front forks (telescopic, disassembly, oil change, seal replacement, spring inspection), rear shock absorber (monoshock, disassembly, oil change, seal replacement, spring adjustment, linkage maintenance).
Braking Systems:
Front and rear disc brakes, hydraulic system (master cylinder, caliper, brake lines), pad inspection and replacement, fluid flush and bleed, rotor inspection.
Electrical Systems:
Ignition system (CDI, stator, coil, spark plug), lighting (if equipped), wiring diagrams, battery (if applicable for ignition), charging system.
Chassis And Bodywork:
Frame inspection, fuel tank, seat, fenders, handlebars, footpegs, exhaust system (pipe, silencer), radiators (if applicable).
Routine Maintenance:
Pre-ride inspections, oil changes (transmission), chain lubrication and adjustment, air filter cleaning/replacement, spark plug inspection, tire pressure checks.
Engine Repair:
Top-end rebuilds, piston/ring replacement, crankshaft inspection/replacement, gearbox inspection/repair, clutch service.
Suspension Service:
Fork oil changes, seal replacements, shock absorber rebuilds, linkage bearing service.
Troubleshooting:
Diagnosis of common starting issues, performance problems, electrical faults, and mechanical failures with step-by-step diagnostic procedures.
General Overhaul:
Complete disassembly and reassembly procedures for major components.
Depth:
Comprehensive: Covers everything from basic maintenance and adjustments to complete engine and component rebuilds, including detailed torque specifications and diagnostic charts.
Engine Type:
Liquid-cooled, 2-stroke, single cylinder, reed valve induction.
Bore X Stroke:
66.4 mm x 72 mm
Compression Ratio:
8.2:1 (with standard 38.5cc combustion chamber volume)
Carburetor:
Keihin PWK38S (38mm) with Power Jet.
Horsepower:
Approx. 47-50 HP (at crank, varies by tuning and dyno)
Torque:
Approx. 28-30 lb-ft (at crank, varies)
Fuel Type:
Unleaded gasoline with 2-stroke oil pre-mix.
Fuel Oil Ratio:
Typically 40:1 or 50:1 depending on oil quality and usage.
Fuel Tank Capacity:
8.0 Liters (2.1 US gallons)
Infotainment System:
N/A (Competition motocross bike, no infotainment features).
Driver Assistance Features:
N/A (Competition motocross bike, no electronic driver assistance).
Connectivity Options:
N/A (Competition motocross bike, no connectivity features).
Ignition Type:
Digital CDI (Capacitor Discharge Ignition) with electronic advance.
Oil Change Transmission:
Every 10-15 hours of riding or annually.
Air Filter Cleaning:
After every few rides, or more frequently in dusty conditions.
Spark Plug Inspection:
Every 10-15 hours.
Top End Rebuild:
Typically every 40-60 hours of aggressive riding, or based on compression loss.
Transmission Oil Type:
Yamaha Yamalube 10W-30 API SG or SAE 10W-30 API SE, SF type transmission oil.
Transmission Oil Capacity:
0.70 Liters (0.74 US qt)
Coolant Type:
High-quality, pre-mixed ethylene glycol-based coolant with corrosion inhibitors (e.g., Yamalube Long Life Coolant or equivalent).
Brake Fluid Type:
DOT 3 or DOT 4 hydraulic brake fluid.
Power Valve Sticking:
The exhaust power valve mechanism can accumulate carbon and become stiff or seize if not regularly cleaned and lubricated, affecting performance.
Clutch Wear:
Aggressive riding can lead to premature clutch plate wear, requiring adjustment or replacement.
Suspension Seals:
Like many motocross bikes of the era, fork seal leaks can occur with heavy use.
Carburetor Jetting:
Proper carburetor jetting is crucial for optimal performance at different altitudes and temperatures; this manual helps in tuning.
Yamaha Motorsports:
Yamaha has a long and successful history in motorsports, particularly in motocross, with the YZ line being a dominant force.
Yz250 2-Stroke Line:
The YZ250 2-stroke has been a staple in motocross for decades, consistently updated to remain competitive. The 2003-2004 models featured significant updates, including revised suspension and engine tuning.
2004 Updates:
For 2004, Yamaha focused on chassis refinements, updated suspension settings, and a revised carburetor for improved low-end power delivery and overall tractability. The styling also received minor tweaks.
Manufacturing Location:
Japan
