Yamaha YZ125T Owner's Service Manual, 1987 (P/N LIT-11626-05-72)

This Yamaha factory Owner's Service Manual covers the 1987 YZ125T, model code 2HG, P/N LIT-11626-05-72. Owner information covers controls, fuel and fluids, pre-operation checks, starting and break-in, cleaning, storage, lubrication, and the race-based maintenance schedule.

PDF Bookmarks, Diagram Detail and Page Count

Search and diagram access: PDF text search is unavailable; embedded bookmarks cover the seven service chapters and their sections, although some front-matter destinations are incorrect or inactive. Small wiring labels and connector details are readable when enlarged.

The ZIP contains one printable English PDF with 208 pages total, including the illustrated-symbol key and index, four blank divider pages, three memo pages, and the Yamaha back cover.

Two-Stroke Engine, Chassis and Ignition Service

These selected service topics outline the repair coverage rather than reproduce the complete table of contents.

• Fuel and exhaust service: Main-jet replacement, carburetor teardown and 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: Step-by-step procedures for cylinder-head, cylinder and piston removal, carbon removal, dimensional inspection, piston and ring checks, and power-valve assembly and alignment.

• Bottom-end and drivetrain work: Engine removal, crankcase separation, crankshaft installation, clutch and primary-drive service, kick axle and shift shaft repair, transmission inspection, and YPVS governor overhaul.

• Cooling-system diagnosis: Radiator-cap and cooling-system pressure tests, hose and core inspection, and water-pump removal, inspection and installation.

• Chassis and brake repair: Wheel bearings, spokes, chain and sprockets, rear drum-brake service, front caliper and master-cylinder overhaul, brake bleeding, steering bearings, and swingarm adjustment.

• Suspension service and setup: Front-fork teardown, seal and bushing replacement, oil and damping adjustments, spring and spacer selection, rear-shock settings, and B.A.S.S. control-cable service.

• Ignition troubleshooting: CDI magneto removal, ignition-timing adjustment, circuit and wiring diagrams, and no-spark or intermittent-spark diagnosis using component resistance checks.

The internal repair and measurement procedures make this manual best suited to experienced owners or mechanics with precision measuring equipment and access to the listed service tools.

Engine Measurement and Service Tooling

The engine repair chapters pair assembly drawings with measurement data to help evaluate parts before reuse. Inspection coverage includes cylinder-head warpage, cylinder bore wear and geometry, piston-to-cylinder clearance, and ring side clearance and end gap.

Bottom-end checks cover crankshaft runout, connecting-rod play, clutch wear, transmission-shaft runout, shift-fork condition, gear dogs, bearings, and oil seals. Assembly guidance identifies transmission checks before case-fastener tightening and crankshaft and transmission checks after assembly.

Engine tools: The special-tool section identifies crankcase separator YU-01135, crankshaft installer components YU-90050/YU-90062, and rotor holder and puller YU-01235/YM-01189.

Fork tools: Seal and bushing tools YM-08020/YM-33963 and fork cylinder tools YM-33962/YM-01326 are identified for the corresponding chassis procedures.

During cylinder removal, the manual warns that coolant left in the crankcase can rust the bearings. Water-pump service identifies the oil seal's WATER SIDE marking and warns that careless impeller-shaft installation can damage the seal lip or dislodge its spring.

B.A.S.S., Suspension and Tuning Limits

The Brake Actuated Suspension System (B.A.S.S.) reduces rear-shock compression damping when the rear brake is applied. Its control-cable adjustment depends on correct brake-pedal height and free play.

Damper limits: Front-fork service treats the variable damper as an assembly rather than providing internal disassembly. Internal rear-shock cylinder and reservoir repair is also outside the scope: Yamaha warns against opening the nitrogen-charged unit, specifies damper-assembly replacement for leakage or damage, and reserves shock disposal for Yamaha dealers.

Detailed carburetor and suspension tuning is referred to the separate Race Preparation and Tuning Manual 90894-13400, which is not included. This manual supplies baseline carburetor settings, replaceable jet data, optional fork-spring and spacer charts, rear-shock spring choices, and sprocket combinations.

Radiator service carries a specific warning about scalding hot, pressurized coolant.

Ignition Diagnosis and Factory Reference Material

Electrical repair focuses on the CDI magneto ignition and engine-stop circuit. The troubleshooting path uses spark, ignition-coil, source-coil, pickup-coil, connection, and CDI checks, with Yamaha Pocket Tester YU-03112 identified for electrical measurements.

The YZ125T Circuit Diagram and YZ125T Wiring Diagram show component connections and wire-color codes. Electrical coverage does not extend to battery, lighting, or charging-system service.

Other visual references include the Cable Routing Diagram and exploded assemblies for Carburetor and Reed Valve, Crankshaft, Transmission and Shifter, and Front Fork.

The appendix adds troubleshooting tables for starting, high-speed performance, overheating, coolant loss, transmission, clutch, brakes, steering, wheels, frame, and swingarm. Component-specific torque specifications, fluid and lubricant data, wear limits, clearances, and electrical test values support the repair sections.

Selected Float-Height and Ignition-Coil Data

For the Mikuni TM34SS carburetor, specified left-float height is 11–13 mm with the carburetor inverted, the float-bowl gasket removed, and the float arm resting on the needle valve without compressing it.

Ignition-coil test data lists primary resistance of 0.20–0.24 Ω and secondary resistance of 4.0–4.8 kΩ, both at 20°C (68°F).