1991 Yamaha YZ125B Owner's Service Manual (P/N LIT-11626-07-55)

• Official Yamaha publication: YZ125B Owner's Service Manual for the 1991 YZ125(B)/LC 3XJ build, P/N LIT-11626-07-55, 1st Edition, June 1990; Yamaha cover code 3XJ-28199-10; a 261-page PDF with a seven-chapter contents structure and embedded section bookmarks.
• Engine repair: Covers engine removal and installation, cylinder-head/cylinder/piston inspection, power-valve and YPVS governor service, clutch repair, crankcase splitting, crankshaft installation, and six-speed transmission and shift-cam work.
• Fuel and cooling systems: Details TM35SS carburetor and reed-valve disassembly, metering-component and float inspection, radiator and water-pump service, premix and transmission-oil procedures, and race-specific carburetor tuning.
• Chassis service: Includes wheel-bearing work, front and rear hydraulic disc-brake overhaul and bleeding, inverted-fork disassembly and seal replacement, steering-bearing adjustment, swingarm and relay-arm inspection, and rear-shock spring and setup procedures.
• Electrical diagnosis: Provides the electrical-component layout, color-coded wiring diagram, no-spark fault-isolation chart, spark-gap test, engine-stop switch checks, ignition-coil testing, CDI magneto testing, and CDI-unit diagnosis.
• Factory reference data: Supplies component-specific torque tables, wear limits, clearances, capacities, electrical test specifications, cable-routing drawings, setting-part numbers, and Yamaha special-tool references for engine, chassis, and electrical repair.

Model and Configuration Coverage

This is a model-specific manual, not a broad multi-year YZ125 book. Yamaha identifies 3XJ1 for EUR, 3XJ2 for USA, AFR, CSA, CDN, and ZA, and 3XJ4 for AUS and NZ. The identification section gives VIN starts JYA3XJW0*MA008101 for USA/CDN and JYA3XJT0*MA020101 for AUS, together with regional frame and engine starting numbers 3XJ-000101, 3XJ-008101, and 3XJ-020101.

Those distinctions carry into the specifications. The manual separates European carburetor and fork settings, identifies regional spark-plug and fuel requirements, and provides the matching standard and optional setting parts. That helps prevent selecting a jetting baseline, suspension setup, or replacement component for the wrong 3XJ configuration.

Factory Repair Depth

The engine chapter supports a full tear-down. It moves from the exhaust, radiator hoses, TM35SS carburetor, reed valve, cylinder, piston, and power valve through the clutch, kick/shift mechanism, YPVS governor, CDI magneto, engine removal, crankcase and crankshaft, transmission, and water pump. Each assembly is organized around removal notes, inspection criteria, assembly orientation, torque callouts, and final operating checks rather than a parts-only exploded view.

Yamaha's special-tool section also shows the intended repair depth and gives separate USA/CDN and 90890- tool numbers. Examples include the Crankcase Separating Tool YU-01135 / 90890-01135, crankshaft installing tools, Fork Seal Driver YM-38853 / 90890-01442, Yamaha Pocket Tester YU-03112 / 90890-03112, and Ignition Checker YM-34487 / 90890-06754.

Chassis coverage goes well beyond routine adjustment. Both disc brakes include master-cylinder and caliper disassembly, inspection, reassembly, and bleeding. The fork section covers the damper rod, check valve, inner and outer tubes, slide metal, oil and dust seals, oil filling, and left-to-right level checks. Steering bearings, swingarm bearings, relay-arm components, and swingarm side-clearance shims are also addressed.

The nitrogen-charged rear shock is an important scope limit. Yamaha covers removal, spring replacement, external inspection, installation, damping and preload adjustment, and dealer-only disposal, but specifically instructs the owner not to disassemble the shock cylinder or separate nitrogen tank.

Two manual-specific cautions are especially useful during repair. Bottoming the inner fork tube while pulling the fork apart can damage the oil-lock piece. Forcing the crankcase halves apart or driving the crankshaft out with a hammer risks damage to the cases, mating surfaces, or shaft; the factory separating and installing tools are specified for that work.

Tuning, Diagrams, and Reference Data

The tuning chapter covers carburetor-component working ranges and rich/lean symptom checks, spark-plug reading, secondary gearing selection, fork oil-level and spring effects, rebound and compression damping, rear spring preload, front-to-rear suspension balance, track-condition troubleshooting, handlebar position, and a setting-record table.

Representative visual references include the cable-routing diagram, electrical-component and wiring diagram, ignition-system fault-isolation flowchart, carburetor working-range chart, exploded crankcase/transmission and brake assemblies, and front-fork and rear-shock adjustment illustrations. Separate setting-parts tables list Yamaha numbers for alternate jets, throttle valves, jet needles, sprockets, fork and shock springs, handle crowns, and lower holders. The related factory data covers torque specifications, wear limits, fit and clearance checks, fluid capacities, ignition test values, and adjustment specifications where each repair procedure calls for them.