1986 Yamaha YZ125S Service Manual (P/N LIT-11626-05-13)

• Covers the 1986 Yamaha YZ125S competition motorcycle, identified by model code 1LX, VIN beginning *JYA1LX00GA000101, and engine number beginning 1LX-000101**.
• Documents Mikuni TM34SS carburetor service, reed-valve inspection, cylinder and piston repair, YPVS power-valve and governor work, and water-pump and cooling-system checks.
• Follows engine removal, crankcase separation and assembly, crankshaft service, clutch and kick-starter repair, and inspection of the six-speed transmission, shift forks, guide bars, and shift cam.
• Provides front disc-brake repair and bleeding, steering-head service, front-fork disassembly and seal replacement, plus wheel, drive-chain, and swingarm/linkage work.
• Explains MonoCross “De Carbon” rear-shock settings and removal, spring installation, and Brake Actuated Suspension System (B.A.S.S.) cable replacement and adjustment; internal shock overhaul is outside this manual’s scope.
• Includes the ignition wiring diagram, spark and component test charts, electrical specifications, labeled connector/component views, and two-page cable-routing diagrams.
• Identifies the original Yamaha Motor Corporation, U.S.A. publication as P/N LIT-11626-05-13, 1st Edition, August 1985, presented here as a 133-page PDF.

The YZ125S Owner’s Service Manual combines operating and routine-maintenance information with factory procedures for major engine and chassis repair. Its technical depth and reliance on measuring equipment and model-specific service tools make it best suited to experienced owners and mechanics who are comfortable working from specifications and exploded assemblies.

Engine and Transmission Detail

The engine chapter covers preparation, removal, disassembly, inspection, repair, and reassembly for the 123 cc liquid-cooled premix two-stroke. The carburetor section supplies the standard setup, optional tuning-part numbers, float inspection, and cleaning guidance. Separate sections address the reed valve and muffler, along with wear checks and assembly data for the cylinder head, cylinder, piston, ring, and power valve. Detailed carburetor tuning is referred to Yamaha’s separate Race Preparation and Tuning Manual.

Bottom-end coverage reaches well beyond routine maintenance. It shows how the cases are split, where the bearings and oil seals fit, and how the crankshaft and transmission assemblies are inspected and installed. Gear and shift-system work covers the mating dogs, three numbered shift forks, guide bars, shift cam, segment, and stopper mechanism before the cases are closed. The appendix adds symptom-based fault tables for gears that slip, skip, fail to select, or leave the shift pedal unable to return.

Factory tools are identified by part number, including the YU-01135 crankcase separating tool and the YU-90058, YU-90060, and YU-90062 crankshaft installing set. The manual also calls out the YM-91042 clutch holder, YM-33962/YM-01326 fork-cylinder tools, YM-08020 fork seal and bushing service tool, YU-03097/YU-01256 dial gauge and stand, and YU-03112 pocket tester.

During crankcase separation, Yamaha warns against forcing a hung case half or striking a gasket-mating surface, which can damage the case. In the fork section, bottoming the inner tube while extracting the guide bushing can damage the oil-lock piece.

Chassis, Suspension, and Brakes

Wheel and drivetrain procedures address spokes, bearings, sprockets, drive-chain wear, alignment, and wheel installation. Brake coverage includes rear-drum inspection and front hydraulic caliper and master-cylinder repair, pad replacement, bleeding, and disc inspection. The steering and fork material covers steering-head bearings, fork oil replacement, complete disassembly, damper-rod service, guide-bushing and seal installation, spring choices, and setup data. Swingarm, relay-arm, and connecting-rod inspection and installation are included as well.

Rear-suspension coverage is specific to the gas-charged MonoCross “De Carbon” system and its B.A.S.S. linkage to the rear brake pedal. It provides preload, spring, rebound, compression, cable, and installation settings, but directs full suspension-tuning detail to the separate tuning manual. The pressurized shock body is not disassembled here, and Yamaha reserves its disposal procedure for dealers.

Diagrams, Diagnosis, and Specifications

Exploded views appear ahead of disassembly sections to clarify part order and orientation. Representative drawings include the carburetor, cylinder-head/YPVS assembly, clutch, six-speed transmission and shifter, front disc brake, and MonoCross rear shock. The ignition schematic and cable-routing views locate the CDI unit, flywheel magneto, engine-stop circuit, brake hose, control cables, and B.A.S.S. cable.

Troubleshooting charts trace hard starting or no-start conditions, poor high-speed performance, overheating, and coolant loss. Separate tables address transmission and shifter faults, clutch slip or drag, steering play, wheel runout, and brake problems. Reference data covers maintenance intervals, approved fuels and lubricants, capacities, wear limits, clearances, gear ratios, engine and chassis torque specifications, suspension setup ranges, and CDI magneto and ignition-coil test values.

The safety notes are specific to this machine: release fork air before disassembly, keep oil and contamination away from brake components, and never open or heat the nitrogen-charged rear shock. Use the complete factory procedures and cautions before applying specifications to a repair.