Honda TRX400EX Fourtrax 1999–2004 Service Manual (P/N 61HN100)

• Official OEM publication (P/N 61HN100) covering the 1999-2004 Honda TRX400EX Fourtrax 397cc single-cylinder dry-sump engine.
• Features 47 exploded-view diagrams and 14 technical torque-value tables for frame, engine, and electrical sub-assemblies.
• Includes specific decompressor assembly details and high-altitude carburetor tuning parameters frequently omitted from standard owner literature.
• Transmission overhaul, hydraulic disc brakes, electric starter, swingarm linkage, charging system.
• Complete engine teardown, valve clearance adjustment, brake bleeding procedures, cable routing, emission controls, troubleshooting flow charts.

Structural and Maintenance Scope

This 265-page PDF factory service manual provides complete reference material for the major vehicle systems of the Fourtrax. It starts with VIN, engine, and frame serial number locations before moving into comprehensive teardown and assembly breakdowns. Detailed sections document the cylinder head, valve train, cylinder and piston assemblies, clutch, gearshift linkages, and alternator components. Crucially, the manual provides full procedures for crankcase separation and transmission rebuilding—a complex repair typically reserved for professional shops. You will also find electrical schematics covering the ignition, charging, and lighting systems, alongside essential wiring diagrams and cable & harness routing diagrams. For electrical faults, the dedicated troubleshooting flow charts streamline the diagnostic process.

Additional chapters cover the lubrication system, detailing oil pump inspections and strainer maintenance protocols. The fuel section breaks down the carburetor configuration, including jetting specs for both standard and high-altitude riding conditions. For the chassis, the manual outlines wheel, front and rear suspension, and steering system care. It also delivers step-by-step guides for hydraulic disc brake servicing, including fluid replacement, caliper rebuilds, and proper brake bleeding procedures that save expensive trips to the dealership. By providing factory-calibrated specifications and precise teardown sequences, this publication prevents costly reassembly errors and eliminates trial-and-error diagnostics during complex overhauls.

Technical Preview and Reference Values

This publication lists specific clearances and capacities for the 397cc powerplant. For example, intake valve clearance is set at 0.10 mm (0.004 in), while the exhaust valve clearance is specified at 0.12 mm (0.005 in). Engine oil capacity is documented at 1.8 liters after draining. The carburetor uses a #148 main jet as the standard factory setting for low-altitude operation, and piston-to-cylinder clearance falls within the standard 0.015–0.050 mm range. Essential torque values are fully documented, including 44 N-m for cylinder head nuts and 108 N-m for the clutch center lock nut. Visual aids like valve seat cutter views and emission control schematics further support precise machine work.

When working on the TRX400EX RFVC engine, technicians must be highly precise during valve clearance checks. Rotating the crankshaft past the "T" mark activates the automatic one-way decompressor system, which leads to wildly inaccurate clearance readings. Additionally, proper orientation of the oil filter is critical; installing it backwards blocks the flow in the dry-sump system and causes immediate, severe engine damage.

Working on ATVs involves inherent mechanical risks. Proper maintenance requires a safe, well-ventilated workspace, and anyone attempting extensive engine work should follow standard shop safety practices or consult a certified mechanic.