Welcome to the comprehensive guide for the YFZ450, a groundbreaking sport ATV that defined a new era in off-road performance. Produced for the 2003 and 2004 model years, this first-generation YFZ450 is renowned for its potent 449cc liquid-cooled, 4-stroke engine, derived from cutting-edge sportbike technology. This powerful machine, featuring a 5-valve DOHC configuration and a Keihin FCR carburetor, delivers exceptional horsepower and torque, making it a dominant force on the track and trails. Commitment to engineering excellence is evident in the YFZ450's robust steel chassis, advanced suspension, and powerful hydraulic disc braking system, all contributing to its legendary handling and reliability. This workshop repair service manual, written in English, provides factory-level information essential for maintaining and repairing your YFZ450. It covers all models, engines, and trim types, offering detailed procedures for every system, including the engine top end, fuel system, cooling system, clutch, lubrication, drivetrain, wheels, tires, brakes, suspension, steering, frame, and electrical system. Within its hundreds of pages, you will find comprehensive diagnostics, step-by-step disassembly and reassembly instructions, cleaning procedures, wiring diagrams, and essential safety guidelines. This manual is an indispensable resource for owners and technicians alike, enabling efficient and accurate service to keep your YFZ450 performing at its peak.