2001 Yamaha YZ125N Service Manual (P/N 5MV-28199-30)

• Model coverage: Applies to the 2001 YZ125N1 for the USA, YZ125LC for Europe, and YZ125(N) for Canada, Australia, New Zealand, and South Africa. The corresponding model codes are 5MV1, 5MV2, and 5MV4, and the motorcycle is identified as a closed-course competition machine.
• Engine repair: Covers the TMX38SS carburetor and reed valve, cylinder head, cylinder, piston and power valve, YPVS governor, cooling system and water pump, CDI magneto, complete engine removal, crankcase separation, and crankshaft service.
• Drivetrain work: Details clutch and primary-driven-gear removal, inspection, and assembly; kick and shift mechanisms; and internal service of the six-speed constant-mesh transmission, shift cam, and shift forks.
• Chassis service: Includes wheel and hub work, front and rear brake caliper and master-cylinder repair, hydraulic bleeding, front-fork disassembly and seal replacement, steering-head service, swingarm and linkage inspection, and rear Monocross shock removal and inspection.
• Factory checks: Provides maintenance schedules plus cooling-system pressure tests, brake inspection and adjustment, wheel and chain checks, ignition-timing checks, and component-specific wear limits, clearances, fluid data, and torque tables.
• Race tuning: Explains region-specific carburetor jetting, atmospheric and symptom-based fuel-mixture changes, final-drive ratio selection, fork oil and spring setup, compression and rebound damping, rear spring preload, and suspension symptom charts.
• Document depth: A 251-page PDF with job-sequence charts, cable-routing and ignition wiring diagrams, engine and chassis exploded views, specialty-tool references, and inspection criteria throughout.

Coverage and Intended Use

This Yamaha Owner's Service Manual is specific to the 2001 5MV-series YZ125 configurations listed above; it is not a general guide for other YZ125 model years. Early chapters cover controls, premix fuel and oil, starting and break-in, pre-operation checks, race-oriented maintenance intervals, and storage. The repair chapters use a step-by-step factory format but assume that the reader can measure parts, interpret service limits, and use the specified pullers, holders, gauges, and electrical test equipment. That makes the manual best suited to experienced owners and mechanics already comfortable with two-stroke motorcycle repair.

Regional distinctions matter. The specification and tuning tables separate European carburetor calibration and suspension springs from the setups used in the USA, Canada, South Africa, Australia, and New Zealand. Before selecting parts or procedures, the machine can be matched using the vehicle or frame number, engine number, and under-seat model label.

Engine and Drivetrain Detail

The liquid-cooled premix two-stroke uses Yamaha's YPVS power-valve system, a wet multi-disc clutch, and a six-speed gearbox. Factory repair coverage goes well beyond routine service: exploded layouts and job charts support top-end disassembly, power-valve removal and inspection, piston and ring measurement, cylinder and head checks, crankshaft runout inspection, crankcase splitting and reassembly, and transmission gear, dog, axle, shift-cam, and shift-fork inspection.

The TMX38SS section covers carburetor disassembly, float and fuel-level inspection, reed-valve checks, regional jetting parts, and reassembly. Yamaha treats the venturi block and main nozzle as a non-serviceable assembly; separating them can reduce carburetor performance. That caution is particularly useful when evaluating a used carburetor or planning a rebuild.

Chassis, Ignition, and Setup

Brake coverage includes pad service as well as caliper and master-cylinder disassembly, seal inspection, reassembly, and system bleeding. The fork chapter contains full internal service, oil-level setting, damper-rod work, and seal installation. Rear-shock coverage is deliberately narrower: the nitrogen-charged cylinder and reservoir are not to be disassembled, and shock disposal is reserved for Yamaha dealers.

The electrical chapter is focused on ignition rather than a street-bike lighting or charging system. Its fault-isolation sequence works through spark testing, couplers and leads, the ignition coil, F4SS00 magneto source and pickup coils, and the 5MV-00 CDI unit. Race-setup charts then connect engine response to throttle position and carburetor circuits, while chassis tables relate track symptoms to gearing, tire pressure, spring choice, oil level, preload, and damping changes.

Special Tools and Service Cautions

Listed Yamaha tools include the YU-1135-A/90890-01135 crankcase separator, YM-1189/90890-01189 flywheel puller, YU-24460-01/90890-01325 radiator-cap tester, YM-1423/90890-01423 damper-rod holder, YM-01442/90890-01442 fork-seal driver, and YU-3112-C/90890-03112 pocket tester. U.S./Canadian and international tool numbers are shown side by side where applicable.

The manual warns against hammering the crankshaft or forcing crankcase separation, practices that can damage the cases, crankshaft, or bearings. It also calls for care around hot coolant, brake fluid, and the pressurized rear-shock reservoir. These cautions define the equipment and experience expected for deeper repair work without turning the manual into a substitute for safe workshop practice.