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

The 1986 Yamaha YZ125S Owner’s Service Manual combines operating and routine-maintenance information with factory procedures for major engine and chassis repair. This is the U.S. publication, P/N LIT-11626-05-13, 1st Edition, August 1985; the original cover also carries P/N 1LX-28199-10.

PDF Bookmarks and Diagram Detail

Search, navigation, and wiring labels: PDF text search is not available. Embedded bookmarks cover the chapters and individual service sections. Small wire-color codes and connector labels in the ignition diagram are readable with zooming.

The ZIP download contains one printable English PDF. Its 133 pages include the original cover, four introductory/index pages, one blank page, and two closing warranty and noise-regulation pages.

Two-Stroke Engine, Chassis, and Ignition Service

The following highlights are a selection of the manual’s contents. Other topics include starting and break-in procedures, cleaning, and storage preparation.

• Fuel and intake: Mikuni TM34SS carburetor disassembly, cleaning, float inspection, reed-valve checks, and muffler maintenance.

• Top-end repair: Cylinder-head, cylinder, piston, and ring inspection, plus YPVS power-valve removal, assembly, alignment, and governor service.

• Engine and transmission: Engine removal, crankcase separation, crankshaft inspection and installation, clutch and kick-starter repair, and transmission and shifter checks.

• Brakes and running gear: Front hydraulic brake repair and bleeding, rear-drum inspection, steering-head service, and wheel, sprocket, and drive-chain work.

• Front suspension: Step-by-step fork disassembly and reassembly, damper-rod inspection, guide-bushing and seal replacement, oil replacement, and setup data.

• Rear suspension: MonoCross “De Carbon” shock settings, removal, spring replacement, and Brake Actuated Suspension System (B.A.S.S.) cable replacement and adjustment.

• Electrical diagnosis: Ignition wiring diagram, spark-test charts, source-coil, pickup-coil and ignition-coil tests, connector views, and cable-routing diagrams.

The operating and basic-maintenance sections address owners; internal repair work is suited to experienced owners and mechanics comfortable with measuring equipment, specifications, and model-specific service tools.

Fuel, Cooling, and Top-End Detail

The carburetor section supplies standard setup information and optional tuning-part numbers alongside component inspection guidance. Detailed carburetor tuning is referred to Yamaha’s separate Race Preparation and Tuning Manual.

Top-end coverage includes water-jacket inspection, cylinder-head warpage, cylinder wear, piston sizing, ring clearances, and power-valve condition. Muffler maintenance addresses carbon buildup and silencer packing replacement.

Cooling-system procedures extend to radiator removal, hose inspection, radiator-cap testing, and system pressure checks. The water-pump section addresses the impeller, driven gear, bush, oil seal, and assembly orientation.

Crankcases, Clutch, and Shift Mechanism

Bottom-end procedures show bearing and oil-seal installation and the placement of the transmission assemblies. Inspection covers mating dogs, three numbered shift forks, guide bars, shift-cam grooves, the segment, and stopper mechanism.

Crankshaft coverage includes removal, runout and connecting-rod checks, and installation. Crankshaft repair beyond the specified inspection limits is referred to an authorized Yamaha dealer.

The inspection limits and symptom-based fault tables help connect wear findings with complaints such as gears slipping out, skipped shifts, clutch slip, or clutch drag.

Factory tools are identified by part number, including the YU-01135 crankcase separator, YU-90058/YU-90060/YU-90062 crankshaft installing set, and YM-91042 clutch holder. Ignition timing uses the YU-03097 dial gauge and YU-01256 stand.

Yamaha cautions that forcing a hung crankcase half or striking its gasket-mating surface can damage the case.

Brakes, Forks, and Rear Linkage

Front-brake detail reaches caliper piston and seal inspection, master-cylinder repair, pad replacement, bleeding, and disc checks. Rear-brake material covers shoe thickness, drum condition, and camshaft inspection.

Fork procedures include spring choices, damper-rod service, and seal orientation. Tools include the YM-33962/YM-01326 fork-cylinder tools and YM-08020 seal and bushing tool. Bottoming the inner tube during guide-bushing extraction can damage the oil-lock piece.

Swingarm and linkage coverage includes bearings, bushes, collars, oil seals, the relay arm, and connecting rod. Rear-shock data covers preload, spring selection, rebound, compression, and the B.A.S.S. connection to the rear brake pedal.

Internal shock overhaul is outside this manual’s scope. Full suspension-tuning detail is referred to the separate tuning manual; shock disposal is reserved for Yamaha dealers.

Ignition Diagrams and Fault Tracing

The electrical chapter follows weak-spark and no-spark diagnosis through the engine-stop circuit, CDI magneto, ignition coil, and connections, using the YU-03112 pocket tester.

Illustrations include the ignition schematic, cylinder-head/YPVS exploded view, transmission-and-shifter assembly, front disc-brake assembly, rear-shock assembly, and two-page cable-routing diagram.

Appendix tables address starting difficulties, poor high-speed performance, overheating, coolant loss, transmission faults, steering play, wheel runout, and brake problems. Reference sections provide maintenance intervals, lubricants, capacities, wear limits, gear ratios, torque tables, and electrical test specifications.

The safety notes address fork air pressure, brake contamination, and the nitrogen-charged rear shock. Use the complete factory procedures and cautions before applying specifications to a repair.

A Preview of Piston and Clutch Inspection Limits

Piston-to-cylinder clearance is specified at 0.055–0.060 mm, with a 0.1 mm service limit. The inspection section relates this clearance to measured cylinder bore and piston diameter.

Clutch friction plates have a nominal thickness of 3.0 mm and a 2.7 mm wear limit. Clutch springs have a standard free length of 35.0 mm and a minimum length of 33.0 mm, alongside separate plate-warpage and housing-clearance data.