Yamaha WR400F / WR426F Service Manual, 1998–2002 (P/N 5NG-28199-30)

Yamaha’s WR400F(N) and WR426F(N) Owner’s Service Manual (P/N 5NG-28199-30) covers operation, inspection, maintenance, repair, and tuning for the regional competition configurations listed in the model table.

Searchable PDF and Wiring Diagram Detail

Text search and navigation: Embedded text supports searching across the technical chapters. The 28 PDF bookmarks open language-specific contents pages, where linked entries lead to the corresponding sections. Small wire-color codes and connector labels in the wiring diagram are readable when zoomed in.

One printable, 758-page PDF contains English, French, German, and Spanish material. The total includes the front cover, 22 introductory and contents pages, and one rear publication page.

Engine Rebuilds, Chassis Service and CDI Troubleshooting

This selection of workshop topics highlights the repair and diagnostic coverage. The manual also explains starting and break-in, cleaning, and storage.

• Engine repair: Camshaft removal, valve timing, cylinder-head service, valve and spring inspection, and cylinder and piston inspection and assembly.
• Clutch and transmission: Separate WR426F and WR400F clutch procedures, engine removal, crankcase separation, crankshaft installation, and transmission, shift-cam, and shift-fork inspection.
• Carburetor and tuning: KEIHIN FCR-MX39 disassembly and assembly, float and accelerator-pump checks, jet selection, and carburetor settings by symptom.
• Lubrication and cooling: Engine-oil, filter, and strainer service, trochoid oil-pump inspection, radiator checks, and water-pump removal and assembly.
• Brakes and chassis: Caliper and master-cylinder service, brake bleeding, front-fork disassembly and seal replacement, and steering-head, swingarm, and linkage bearing work.
• Electrical diagnosis: Ignition fault tracing, CDI magneto and ignition-coil tests, neutral-switch inspection, TPS checks and adjustment, and lighting-system diagnosis.
• Diagrams and settings: Wiring, cable-routing, and lubrication diagrams, valve-clearance pad-selection tables, and symptom-based suspension adjustment charts.

This manual suits mechanics and experienced owners with the measuring equipment and special tools needed for the work they plan to tackle.

Routine Maintenance and Engine Rebuilds

The maintenance schedules organize inspection and lubrication by break-in, race frequency, and distance. The regular adjustment chapter covers decompression-lever free play and air-filter service.

With a top end apart on the bench, the component sections bring dimensions, wear limits, tightening data, and assembly references together. Exploded diagrams and job instruction charts identify parts and the extent of removal, with replacement criteria, lubricant and locking-agent callouts, and assembly notes.

The WR426F’s six-spring clutch and the WR400F’s five-spring clutch have different friction-plate arrangements and wear limits. The separate procedures help you select the correct inspection limits for the clutch on the bench.

The engine chapter also covers the balancer, kick axle, shift shaft, and CDI magneto. Crankcase and crankshaft procedures include cautions about uneven case separation and hammering the crankshaft.

FCR-MX39 Service and Tuning

Carburetor inspection covers the float valve and accelerator pump, with float-height and intake-vacuum specifications. Tuning information relates main-jet, pilot-jet, pilot-screw, and needle selection to throttle opening and atmospheric conditions.

Yamaha lists needle and clip settings of OBDRR-4 for the WR426F USA version, OBDQR-4 for other WR426F markets, and OBDRQ-4 for the WR400F. The symptom chart links running complaints to mixture settings and checks for fuel-supply or air-cleaner problems.

Oil Circulation and Cooling-System Work

Dry-sump lubrication diagrams trace the oil tank, strainer, trochoid oil pump, filter, and check ball. They identify feeds to the cylinder head, camshaft and valve lifter, crank pin and piston pin, and the transmission shafts and gears.

Oil-capacity tables distinguish periodic changes, filter replacement, and total fill. The oil filter, water pump, and right crankcase cover share a service section. Cooling-system coverage includes radiator-cap pressure testing and coolant replacement.

Forks, Brakes and Rear Suspension

Front-fork procedures cover damper components, oil seals, and filling and level checks using Suspension Oil 01, with separate regional settings. Chassis references include axle torque tables, chain-slack and tire-pressure specifications, and the DOT 4 brake-fluid requirement.

Rear-shock coverage includes removal, spring and bearing service, inspection, and adjustment. Yamaha specifies replacement of the absorber assembly for oil or gas leaks or a bent or damaged damper rod. Keep the pressurized shock cylinder and nitrogen reservoir intact.

The linkage instructions warn that incorrect connecting-rod nut orientation can bring the nut into contact with the swingarm during suspension travel. Brake work and the pressurized rear shock require the precautions and equipment specified in the manual.

CDI, TPS and Lighting Fault Tracing

The map-controlled CDI uses engine speed and throttle position to determine ignition timing. Ignition diagnostic charts help trace faults through connections, switches, and coils before the CDI unit is replaced.

The WR426F uses CDI unit 5NG-00 for the USA and 5NG-10 for other markets. The WR400F uses 5GS-60. TPS procedures cover coil and variable-resistance checks, replacement and adjustment, and input-voltage testing.

The wiring diagram marks connector differences for markets outside the USA. The lighting flowchart covers bulbs and sockets, the lights switch, the lighting coil, wiring connections, and the regulator.

Special Tools Identified for the Work

Engine tools: Yamaha lists the YU-1135-A / 90890-01135 crankcase separating tool, YU-33270-B / 90890-01362 flywheel puller, and YM-04019 / 90890-04019 valve spring compressor.

Clutch, fork and electrical tools: The manual lists the YM-91042 / 90890-04086 clutch holding tool, YM-01442 / 90890-01442 fork seal driver, and YU-3112-C / 90890-03112 pocket tester. The special-tools section explains the regional numbering conventions.

Sample Valve, Piston, Clutch and TPS Specifications

Both engines use cold valve clearances of 0.10–0.15 mm intake and 0.20–0.25 mm exhaust. Piston-to-cylinder clearance is 0.040–0.065 mm for both engines, with a 0.10 mm service limit.

For clutch spring inspection, the standard free length is 48.4 mm for the WR426F and 44 mm for the WR400F. The TPS coil test specifies 4–6 kΩ at 20°C (68°F).