Yamaha YZ250B 1991 Owner's Service Manual (P/N LIT-11626-07-56)

Yamaha’s Owner’s Service Manual for the 1991 YZ250B closed-course competition model covers operation, maintenance, tuning, and repair of the regional 3XK versions. P/N LIT-11626-07-56, first edition, June 1990.

PDF Bookmarks, Text Search, and Download Format

Navigation and wiring detail: Embedded PDF bookmarks lead to chapters and key subsections. Text search is limited to cover text; the manual body has no searchable text layer. Small wiring-diagram labels are readable with zooming.

The ZIP download contains one printable English PDF with 262 pages, including the original cover, preliminary material, and eight largely blank separator pages.

Two-Stroke Engine, Chassis, and Race-Setup Coverage

The following is a selection of covered work, rather than a complete contents list. Other topics include drive-chain inspection and silencer fiber replacement.

• Fuel system and top end: TM38SS Mikuni carburetor and reed-valve service, plus cylinder-head, cylinder, piston, ring, small-end bearing, and YPVS power-valve repair.

• Bottom end and transmission: Clutch and primary-gear inspection, kick and shift mechanisms, YPVS governor service, CDI magneto removal, crankcase separation, crankshaft service, and transmission assembly.

• Cooling system: Radiator and hose removal, cap and system pressure testing, coolant replacement, and water-pump impeller-shaft and oil-seal service.

• Wheels and hydraulic brakes: Wheel inspection and bearing replacement, caliper and master-cylinder service, piston seals, brake discs, hoses, fluid filling, and air bleeding.

• Steering and suspension: Steering bearings, swingarm and relay-arm inspection, upside-down fork disassembly, seal replacement, oil-level setup, damping adjustment, and rear-shock spring service.

• Ignition and race tuning: CDI wiring and fault isolation, carburetor calibration, spark-plug assessment, gearing selection, suspension troubleshooting, handlebar positioning, and track-setting records.

• Factory reference data: Component torque tables, capacities, lubricants, wear limits, clearances, electrical test specifications, cable routing, regional setting parts, and special-tool references.

The repair sections are suited to experienced two-stroke mechanics with the specified measuring equipment and service tools; Yamaha recommends dealer maintenance for the front fork.

Regional Specifications and Race Setup

Regional specification and setting tables distinguish European gearing, carburetor calibration, fork springs and oil setup, and rear-shock spring and damping data. Separate fuel and spark-plug notes apply to Australia, Canada, and South Africa. These distinctions allow parts and setup data to be matched to the correct 3XK build before repair work begins.

The carburetor working-range chart relates mixture-control components to throttle opening. Suspension symptom-and-remedy charts address handling complaints and front-to-rear balance, while the setting-record table provides a format for recording track tests.

Repair Procedures and Service Limits

Step-by-step instructions for crankcase separation, transmission assembly, hydraulic-brake service, and fork overhaul combine illustrated removal orders with job charts, inspection criteria, and installation notes.

The opening chapters cover controls, premix fuel and oil, pre-operation checks, starting, break-in, torque checks, cleaning, and storage. Race-based maintenance schedules organize regular inspection and lubrication work.

Rear-shock scope: Removal, spring handling, external inspection, adjustment, and dealer-only disposal guidance are covered. Internal damper disassembly and nitrogen recharging are not included; Yamaha warns against opening the cylinder or reservoir.

Electrical scope: The electrical chapter covers the CDI magneto, CDI unit, ignition coil, stop button, leads, and spark plug. Its diagnostic sequence separates connection, switch, coil, and magneto checks before CDI-unit replacement; battery and lighting systems are outside this chapter’s scope.

Special Tools, Drawings, and Repair Cautions

The special-tool index cross-references USA/Canada YM/YU numbers with 90890-series numbers for other markets. Engine tooling includes the crankcase separator YU-01135/90890-01135.

Fork tools include seal driver YM-38853/90890-01442 and damper-rod holder YM-1423/90890-01423. Electrical references specify the Yamaha Pocket Tester YU-03112/90890-03112 and Ignition Checker YM-34487/90890-06754.

Visual references include the Cable Routing Diagram and Electrical Components and Wiring Diagram, plus exploded and sectional views for the cylinder and piston, crankcase and crankshaft, front fork, and front and rear brakes.

The fork section explains how bottoming the inner tube during separation can damage the oil-lock piece, and how a dropped damper rod can damage its internal valve. Crankcase cautions address damage caused by forcing the halves apart or striking their gasket surfaces.

Support the motorcycle securely, release fork air pressure before cap removal, and keep gasoline away from heat, sparks, and open flame.

Selected Piston-Clearance and Clutch-Wear Specifications

Piston fit: The piston-to-cylinder clearance specification is 0.050–0.055 mm, with a service limit of 0.1 mm. The inspection section relates this clearance to measured cylinder bore and piston diameter.

Clutch inspection: Friction-plate thickness is specified at 2.9–3.1 mm, with a wear limit of 2.7 mm. These values accompany the plate-inspection and replacement criteria.