1987 Yamaha YZ125 JYA2HG-Series Service Manual (P/N LIT-11626-05-72)

This Yamaha factory Owner's Service Manual covers the 1987 Yamaha YZ125T, model code 2HG, beginning with VIN *JYA2HG00HA000101 and engine number 2HG-000101. It is Yamaha Literature No. LIT-11626-05-72. The specifications identify the liquid-cooled, single-cylinder two-stroke engine, Mikuni TM34SS carburetor, CDI magneto ignition, and semi-double-cradle frame. This is a 208-page PDF** arranged in seven numbered chapters. Because the repair procedures extend to crankcase splitting, transmission inspection, hydraulic brake work, and fork overhaul, the manual is best suited to experienced owners or mechanics with precision measuring equipment and access to the listed service tools. Owner-level material covers controls, fuel and fluids, pre-operation checks, starting and break-in, cleaning, storage, lubrication, and the race-based maintenance schedule.

• Fuel and exhaust service: Covers main-jet replacement; carburetor removal, disassembly, cleaning, float-height and fuel-level checks; reed-petal and stopper inspection; exhaust-pipe service; and silencer packing replacement.
• Top-end and power-valve repair: Details cylinder-head, cylinder, and piston removal; carbon removal; dimensional inspection; piston and ring checks; power-valve disassembly, indexing, and final movement checks.
• Bottom-end and drivetrain work: Walks through engine removal, crankcase separation, crankshaft removal and installation, clutch and primary-drive service, kick axle and shift shaft repair, transmission inspection, and YPVS governor overhaul.
• Cooling and ignition diagnosis: Includes radiator-cap and cooling-system pressure tests, hose and core inspection, water-pump teardown, CDI magneto removal, ignition-timing adjustment, and no-spark or intermittent-spark fault isolation.
• Chassis and brake repair: Extends from wheel bearings, spokes, chain, and sprockets to rear drum-brake service, front caliper and master-cylinder overhaul, brake bleeding, steering-head bearings, and swingarm side-clearance adjustment.
• Suspension service and setup: Addresses front-fork disassembly, seal and bushing replacement, fork oil, air, and damping adjustments, optional spring and spacer selection, rear-shock spring and damping setup, and B.A.S.S. control-cable adjustment.
• Factory reference material: Provides component-specific torque specifications, fluid and lubricant data, wear and clearance limits, electrical test values, setting charts, troubleshooting tables, exploded assemblies, cable-routing views, and wiring information.

Engine Measurement and Service Tooling

The engine repair chapters pair assembly drawings with the measurement data needed to evaluate parts before reuse. Coverage includes cylinder-head warpage, cylinder bore wear and geometry, piston-to-cylinder clearance, ring side clearance and end gap, crankshaft runout, connecting-rod play, clutch wear, transmission-shaft runout, shift-fork condition, gear dogs, bearings, and oil seals.

The special-tool section identifies the crankcase separator YU-01135, crankshaft installer YU-90050/YU-90062, rotor holder and puller YU-01235/YM-01189, fork seal and bushing tools YM-08020/YM-33963, fork cylinder tools YM-33962/YM-01326, and Yamaha Pocket Tester YU-03112. The bottom-end procedure calls for checking transmission operation before the case fasteners are torqued, then checking both the crankshaft and transmission again after assembly.

During cylinder removal, the manual warns that coolant allowed to remain in the crankcase can rust the bearings. The water-pump section specifies the oil seal's WATER SIDE orientation and warns that careless impeller-shaft installation can damage the seal lip or dislodge its spring.

B.A.S.S., Electrical Coverage, and Scope Limits

The chassis chapter covers the YZ125T's Brake Actuated Suspension System (B.A.S.S.), which links rear-brake operation to rear-shock compression damping. It documents control-cable adjustment, removal, inspection, and reinstallation alongside rear-shock spring, preload, rebound, and compression settings. Internal damper repair is outside this manual's scope: Yamaha warns against opening the nitrogen-charged shock cylinder or reservoir, specifies replacement of a leaking or damaged damper assembly, and marks shock-disposal work for Yamaha dealers only. Radiator work likewise carries a specific hot, pressurized-coolant warning.

Electrical repair is limited to the CDI magneto ignition and engine-stop circuit. The chapter supplies a YZ125T circuit diagram and a staged troubleshooting path using spark, ignition-coil, source-coil, pickup-coil, connection, and CDI checks; it does not cover a battery, lighting, or charging system.

Detailed carburetor and suspension tuning is referred to Yamaha's separate Race Preparation and Tuning Manual 90894-13400. This PDF still supplies baseline carburetor settings, replaceable jet data, optional fork-spring and spacer charts, rear-shock spring choices, sprocket combinations, and the standard inspection and adjustment procedures.

Diagrams and Appendix Data

Named visual references include the YZ125T Circuit Diagram, YZ125T Wiring Diagram, Cable Routing Diagram, and exploded assemblies for the Carburetor and Reed Valve, Crankshaft, Transmission and Shifter, and Front Fork. The appendix adds fault tables for hard starting, poor high-speed performance, overheating, low coolant, transmission and shifter problems, clutch faults, brakes, steering, wheels, frame, and swingarm.

The PDF starts with the illustrated-symbol key and index, then proceeds directly to Chapter 1; no cover/title, copyright, or introductory pages precede that point. All seven numbered service chapters are present through the appendix wiring diagram, followed by memo pages and the Yamaha back-cover page.