Category:
Motocross Racing / Performance Off-Road Motorcycle
Target Audience:
Motocross racers, off-road enthusiasts, mechanics, and owners of the 1999 Yamaha YZ250.
Key Competitors:
Honda CR250R, Kawasaki KX250, Suzuki RM250.
Engine Type:
Liquid-cooled, 2-stroke, reed valve induction.
Suspension Technology:
Advanced KYB AOS (Air Oil Separate) front forks and Linkage-Tuned Monoshock rear suspension for superior handling.
Power Delivery:
Renowned for its potent and tractable powerband characteristic of Yamaha's 250cc two-stroke engines.
Engine:
Complete engine tear down, rebuild, diagnosis, tuning, and specifications.
Transmission:
Clutch, gearbox, shifting mechanism inspection, repair, and adjustments.
Suspension:
Front forks (KYB AOS type), rear shock absorber (Monoshock with Linkage) maintenance, rebuild, and settings.
Brakes:
Front and rear hydraulic disc brake systems, caliper and master cylinder service, fluid replacement.
Electrical:
Ignition system, lighting, wiring diagrams, troubleshooting electrical faults, CDI unit.
Fuel System:
Carburetor (Keihin PWK 38S AS) disassembly, cleaning, jetting, and fuel tank/lines.
Cooling System:
Liquid-cooled system, radiator, water pump inspection and service.
Chassis Frame:
Frame inspection, steering head bearings, swingarm, linkage bearings.
Bodywork:
Plastic body panels, seat, handlebars, footpegs, removal, and installation.
Wheels Tires:
Wheel bearing inspection, spoke tensioning, tire changing, and rim maintenance.
Maintenance:
Regular scheduled maintenance, lubrication points, filter changes, spark plug replacement.
Troubleshooting:
Diagnostic procedures for common running issues, starting problems, performance degradation.
Repair:
Detailed step-by-step instructions for overhauling engine components, suspension, brakes, etc.
Rebuild:
Complete engine rebuild procedures, including crankshaft, piston, and cylinder.
Specifications:
Torque specifications for all fasteners, critical measurements, clearances.
Assembly:
Detailed instructions for reassembling components after service or repair.
Cleaning:
Procedures for cleaning specific components like the carburetor and air filter.
Depth Level:
Advanced diagnostics and complete overhaul procedures suitable for experienced mechanics and serious enthusiasts.
Diagrams Illustrations:
Includes numerous detailed illustrations, exploded views, and part diagrams to aid in identification and repair.
Pictorial Guidance:
Features pictures to guide users through complex procedures.
Engine Type:
Liquid-cooled, 2-stroke, single-cylinder, reed valve induction.
Bore X Stroke:
66.4 mm x 72 mm
Compression Ratio:
8.8:1 (Nominal)
Ignition:
CDI (Capacitor Discharge Ignition)
Horsepower Est:
Approx. 45-48 HP (at crankshaft, factory settings)
Torque Est:
Approx. 30-33 lb-ft (at crankshaft, factory settings)
Carburetor Model:
Keihin PWK 38S AS
Fuel Type:
Unleaded gasoline (91 octane or higher recommended)
Fuel Oil Mixture:
50:1 ratio of premium 2-stroke engine oil to gasoline
Oil Change Interval:
Refer to manual for specific transmission oil change intervals based on usage hours/conditions.
Air Filter Cleaning:
Recommended after every few rides or as per manual guidelines, especially in dusty conditions.
Spark Plug Replacement:
Recommended at specified intervals or when performance degradation is noted (refer to manual for specific plug type and gap).
Piston Ring Replacement:
Recommended at specific hour intervals or when compression testing indicates wear (refer to manual for exact figures).
Transmission Oil Type:
Yamaha Gear Oil 10W-30 or equivalent API GL-4 specification.
Coolant Type:
Yamaha's recommended permanent type antifreeze coolant (ethylene glycol base) mixed with distilled water (50/50 ratio).
Brake Fluid Type:
DOT 3 or DOT 4 hydraulic brake fluid.
Common Wear Items:
Clutch plates, piston rings, top-end bearings (crankshaft/rod), wheel bearings, steering head bearings, suspension seals.
Carburetor Tuning:
Sensitivity to jetting changes based on altitude, temperature, and modifications. Regular cleaning and proper jetting are crucial.
Suspension Maintenance:
Fork seal leaks and rear shock seal issues can occur with heavy use. Regular servicing of suspension is important.
