Komatsu PC128US-2 / PC138US-2 / PC138USLC-2 Excavator Service Manual (P/N SEBM018417)

This 548-page PDF set contains two service manuals: a 218-page JYM125/125-2 manual, first edition January 2003, from Chongqing Jianshe-YAMAHA Motor Co., Ltd.; and a 330-page YBR125ED 2005 manual, first edition January 2005, from Yamaha Motor Co., Ltd., with cover code 3D9-F8197-E0. These manuals were written for Yamaha dealers and service personnel with existing knowledge of motorcycle mechanics and repair techniques.

• Engine disassembly and repair: cylinder-head work, piston and ring inspection, crankshaft and balancer service, and oil-pump overhaul.
• Clutch and transmission procedures: wet multiplate clutch inspection, gear and shift-fork checks, crankcase separation, and internal reassembly.
• Brake and chassis service: JYM front-drum procedures and YBR125ED hydraulic front-brake caliper and master-cylinder overhaul, plus fork seal replacement and steering-head bearing work.
• Mikuni VM22SH carburetor: teardown, cleaning, float and fuel-level adjustment, with air-induction-system inspection in the 2005 volume.
• Electrical fault tracing: ignition, electric-starting, charging, lighting, and signaling tests, plus dedicated YBR125ED carburetor-heater and thermo-switch diagnosis.

Model Coverage and Configuration

The earlier YBR125 publication identifies JYM125/125-2, model code 5VL, with a 123cc engine. The 2005 edition identifies YBR125ED, model code 3D91, with a 123.7cc engine. Both cover air-cooled, four-stroke SOHC singles with electric and kick starting, a five-speed transmission, and chain final drive. Match the motorcycle’s model identification and equipment to the appropriate volume when selecting procedures and specifications.

Front-brake coverage differs between editions. The JYM book documents drum and disc configurations, but its detailed front-brake teardown covers the drum assembly; disc coverage includes inspection, fluid replacement, and bleeding. The ED book covers a hydraulic front disc and rear drum. Rear-shock coverage addresses removal, inspection, and replacement; internal damper rebuilding is outside the documented service scope.

Technical Documentation Breakdown

Each volume has eight chapters covering identification, specifications, periodic maintenance, engine, carburetor, chassis, electrical systems, and troubleshooting. Maintenance content includes valve-clearance adjustment, compression testing, oil-delivery checks, and exhaust-gas/CO testing and adjustment.

The 2005 engine chapter carries repair work through valve-guide replacement and reaming, valve-seat lapping, cam-chain timing, starter-clutch checks, and crankshaft installation. Transmission assembly instructions cover shift-drum and fork positioning and operational checks. Chassis sections also address brake-disc runout, wheel bearings, drive-hub dampers, and swingarm clearance.

Electrical diagnosis goes beyond a wiring overview: circuit-specific fault charts work through switches, coils, relays, and wiring. Starter-motor procedures cover the commutator, brushes, and armature. The ED troubleshooting chapter also addresses shifting and clutch faults, overheating, braking, and handling symptoms.

Specification Highlights and Illustrated References

Reference material includes component torque tables, lubrication points, fluid capacities, standard dimensions and wear limits, carburetor settings, A.C. magneto data, and bulb ratings. Engine lubrication diagrams and cable-routing layouts show oil paths and component placement. Front-fork and front brake caliper exploded views identify assembly parts, while the YBR125ED 2005 wiring diagram supports electrical tracing.

Special-tool lists identify equipment such as crankcase separator 90890-01135, clutch holder 90890-04086, and fuel-level gauge 90890-01312. The ED edition also supplies alternative U.S./Canadian tool numbers.

Factory repair cautions explain how solvents can swell brake piston seals and incorrect bearing-race installation can damage the steering-head pipe. Before undertaking any mechanical work, always verify proper motorcycle stabilization and ensure adequate workspace ventilation.