Positioning:
The Yamaha YZ250 in the early to mid-90s was a dominant force in motocross, known for its potent two-stroke power, agility, and competitive chassis. This manual serves owners and mechanics looking to maintain, repair, and optimize these high-performance machines.
Competitors:
Honda CR250R, Kawasaki KX250, Suzuki RM250.
Features:
Powerful 249cc liquid-cooled two-stroke engine, aggressive suspension, lightweight chassis, strong track record in professional racing.
Engine Service:
Complete engine disassembly, inspection, repair, and reassembly
Transmission Service:
Gearbox, clutch, and shifting mechanism maintenance and repair
Chassis Service:
Frame, suspension (forks, shock), steering, and wheels
Electrical Service:
Ignition system, lighting, wiring diagrams, and troubleshooting
Fuel System Service:
Carburetor tuning and overhaul, fuel tank, and lines
Cooling System Service:
Radiator, coolant flow, and related components (where applicable)
Braking System Service:
Front and rear brake system maintenance, adjustment, and repair
Bodywork And Trim:
Fairings, seat, handlebars, and related body components
Troubleshooting:
Diagnostic charts and common problem resolution
Estimated Pages:
300-500 (typical for this era and scope)
Type:
Factory Service & Repair Manual
Format:
PDF (Digital Download)
Engine Mechanical:
Cylinder head, piston, crankshaft, connecting rod, cylinder bore, valve train (if applicable), seals, gaskets.
Engine Performance:
Carburetion jetting, ignition timing, exhaust system tuning principles.
Powertrain:
Clutch operation, gear ratios, shifting mechanism, final drive.
Suspension And Steering:
Front fork (KYB, Showa typically) internal components, rear monoshock linkage, steering head bearings, wheel bearings.
Braking System:
Hydraulic brake system, caliper maintenance, master cylinder overhaul, pad replacement, fluid flush.
Electrical System:
CDI ignition, stator, lighting coils, spark plug, wiring harness continuity testing, troubleshooting faulty circuits.
Fuel And Air System:
Carburetor model identification (e.g., Mikuni VM series), jet sizes, needle settings, float height, air filter maintenance.
Routine Maintenance:
Oil change recommendations, chain adjustment, air filter cleaning, spark plug inspection, brake adjustment.
Engine Rebuild:
Top-end rebuild (piston, rings, cylinder), bottom-end rebuild (crankshaft, bearings, seals).
Clutch Replacement:
Clutch plate, diaphragm spring, and basket servicing.
Suspension Overhaul:
Front fork seal replacement, fork oil change, rear shock absorber service.
Carburetor Rebuilding:
Complete carburetor disassembly, cleaning, and reassembly with new parts.
Electrical Diagnostics:
Using a multimeter to test components, diagnosing wiring faults.
Wheel And Tire Service:
Tire changing, wheel balancing, spoke tensioning.
Chassis Alignment:
Steering head bearing adjustment, wheel alignment checks.
Depth:
Comprehensive, covering everything from basic maintenance to full engine and chassis overhauls. Includes detailed torque specifications, clearances, and diagnostic procedures.
Engine Type:
Liquid-cooled, 2-stroke, reed valve inducted
Bore X Stroke:
Typically 68.0 mm x 68.4 mm (check specific year)
Compression Ratio:
Typically around 7.6:1 (check specific year)
Cylinder Arrangement:
Single-cylinder
Horsepower Estimate:
Approx. 45-50 hp at crank (estimate, depends on specific tuning and year)
Torque Estimate:
Approx. 28-30 lb-ft at crank (estimate)
Carburetor Type:
Mikuni VM Series (e.g., VM38SS)
Fuel Type:
Unleaded gasoline with 2-stroke oil mixture (e.g., 40:1, 50:1, check manual)
Oil Type:
2-stroke engine oil (specific grades recommended in manual)
Transmission Type:
Constant mesh 6-speed
Clutch Type:
Wet, multiple-disc, cable-operated
Gear Ratios:
Detailed gear ratios for each speed available in the manual.
Transmission Oil Type:
Specific gear oil grade (e.g., SAE 80 or SAE 10W-30 transmission oil, check manual for exact specification).
Length:
Refer to specific year specifications (typically around 83-85 inches)
Width:
Refer to specific year specifications (typically around 32-33 inches)
Height:
Refer to specific year specifications (typically around 50-51 inches)
Dry Weight:
Approx. 220-230 lbs (check specific year, varies slightly)
Fuel Tank Capacity:
Approx. 2.4 US gallons (9 Liters)
Cooling System Capacity:
Approx. 1.5 Liters (check manual for exact figure)
Safety Ratings:
Not applicable in the traditional sense for off-road motorcycles; focus is on rider control and machine reliability.
Active Safety:
Effective braking systems (disc brakes front and rear), responsive steering, robust suspension for handling.
Passive Safety:
Durable frame construction, rider protective gear (not part of the vehicle itself, but emphasized in safe operation).
Oil Change Engine:
Recommended every 15-20 hours of riding or annually, check manual for specifics.
Air Filter Cleaning:
After every few rides or as needed based on conditions, check manual.
Spark Plug Inspection:
Every 10-15 hours of riding, check manual for recommended plug type and gap.
Chain Lubrication Adjustment:
Regularly, ideally after every ride or every few rides, check manual for proper slack.
Engine Oil Mixture:
Pre-mix ratio (e.g., 40:1 or 50:1) with specific 2-stroke motorcycle oil. Consult manual for exact ratio and oil type.
Transmission Oil Spec:
SAE 80 or SAE 10W-30 gear oil. Check manual for exact specification and capacity (typically around 0.7-1.0 liter).
Coolant Type:
Ethylene glycol-based coolant, mixed with distilled water (50/50 ratio). Refer to manual for specific freeze protection.
Brake Fluid Type:
DOT 3 or DOT 4 hydraulic brake fluid. Use fresh fluid for flushing.
Common Wear Items:
Piston rings, piston, cylinder bore wear, crank bearings, clutch plates, suspension seals, carburetor jets.
Typical Failures:
Carburetor gumming if fuel sits too long, ignition coil failure, worn wheel bearings, leaky fork seals, clutch cable adjustment.
Carb Cleaning:
Regular carburetor cleaning and proper fuel storage are crucial for optimal performance.
Suspension Care:
Regularly check and replace fork seals and oil to maintain suspension performance and prevent leaks.
Engine Tuning:
Proper jetting for atmospheric conditions and exhaust modifications is key for two-stroke power delivery.
Heritage:
Yamaha has a long and successful history in motorcycle racing, particularly in motocross, with the YZ line being a cornerstone of their performance offerings.
Evolution 1990S:
The YZ250 underwent continuous development throughout the 1990s, with updates to suspension (often introducing USD forks), engine tuning, frame geometry, and styling to remain competitive.
Key Changes 1990-1995:
Introduction of liquid cooling, revised suspension components (e.g., KYB or Showa forks), engine porting and power delivery refinements, chassis stiffening, and aesthetic updates.
Production Focus:
Primarily produced for competitive motocross racing and enthusiast off-road riding.
