1985–1986 Yamaha YZ490S / YZ490 Service Manual (P/N LIT-11626-05-11)

• Yamaha publication P/N LIT-11626-05-11, First Edition, August 1985, presented as a 123-page PDF for the YZ490S, model code 1LV, identified by VIN *JYA1LV00GA000101 and engine starting number 1LV-000101**.
• Covers Mikuni VM40SS carburetor removal and disassembly, Y.E.I.S. air-chamber and reed-valve inspection, muffler maintenance, ignition-timing adjustment, and cylinder/piston top-end service.
• Follows clutch and primary-drive repair through engine removal, crankcase splitting, five-speed transmission and shifter inspection, crankshaft measurement and installation, and crankcase assembly.
• Chassis procedures address wheel and chain service, rear drum-brake inspection, front disc-brake caliper and master-cylinder repair with system bleeding, steering-head service, fork overhaul, and swingarm linkage work.
• Electrical fault-finding covers no-spark and weak-spark diagnosis, engine-stop-switch checks, CDI magneto source and pickup coils, ignition-coil testing, connectors, and CDI-unit isolation.
• Illustrated reference material includes the ignition wiring diagram, cable-routing diagram, and exploded views of the clutch, transmission and shifter, front disc brake, and Monocross/B.A.S.S. linkage.
• Yamaha special-tool callouts identify the YU-01135 crankcase separator, YU-90058/YU-90060/YU-90063/YU-90059 crankshaft installer set, YM-91042 clutch holder, YM-08020 fork service tool, and YU-03112 pocket tester.

This combined owner's and service manual moves from machine setup and race-day maintenance into detailed engine and chassis repair, using Yamaha's sequential procedure format and exploded diagrams ahead of the major disassembly sections. Its repair depth makes it best suited to experienced owners and mechanics who are already comfortable with precision measurement, engine disassembly, hydraulic brake work, and model-specific tools.

Engine, Induction, and Internal Repair

General information records the machine-identification locations, controls, fuel and transmission-oil requirements, pre-operation checks, break-in, cleaning, and long-term storage. A race-oriented maintenance schedule covers the piston and rings, cylinder, clutch, transmission, shift mechanism, carburetor, brakes, suspension, steering head, wheels, chain, and outside fasteners.

The engine chapters are built around the air-cooled, forward-inclined 487 cc two-stroke, with torque induction, a wet multi-disc clutch, and a constant-mesh five-speed transmission. Inspection and specification data cover cylinder-head warpage, cylinder bore condition, piston and ring clearances, crankshaft runout and connecting-rod play, clutch wear and plate warpage, shaft runout, carburetor jetting, and float adjustment. The crankcase section warns that forcing uneven separation or striking gasket-mating surfaces can damage the cases.

Fork, Monocross, Brake, and Swingarm Service

Front-fork data identifies standard and optional springs plus the specified ranges for oil level, tube height, air pressure, and compression damping. The Monocross "De Carbon" rear-shock section records spring choices, preload, rebound and compression settings, gas-pressure specifications, shock removal and installation, and adjustment of Yamaha's Brake Actuated Suspension System (B.A.S.S.).

Brake coverage separates the hydraulic front disc from the mechanical rear drum and supplies wear limits, inspection criteria, component-specific torque tables, approved fluid information, and final adjustment data. The fork section also flags a specific repair trap: bottoming the inner tube during guide-bushing removal can damage the oil-lock piece.

Ignition Diagnosis, Reference Data, and Scope

The ignition chapter distinguishes plug, plug-cap, stop-switch, pickup-coil, source-coil, ignition-coil, connector, and CDI faults, with the applicable resistance and spark-test specifications. Appendix troubleshooting charts cover hard starting, poor high-speed performance, overheating, transmission and shifter faults, clutch problems, steering looseness, wheel runout, brakes, frame, and swingarm concerns. Consolidated tables provide engine, chassis, electrical, torque, wear-limit, clearance, suspension, and conversion data.

Standard carburetor and suspension settings are included, but Yamaha directs full carburetor, front-fork, and rear-shock tuning to the separate Race Preparation and Tuning Manual. Internal rear-shock disassembly is outside this publication's repair scope; it covers setting, B.A.S.S. adjustment, removal, and installation, while shock disposal is marked for Yamaha dealers only. Crankshaft repair after a failed dimensional inspection and wheel-bearing replacement are also referred to a Yamaha dealer.

Because the YZ490S is a competition machine with flammable premix fuel, hydraulic brakes, and pressurized fork and shock assemblies, service should be performed with stable support, ventilation, the correct tools, and the manual's component-specific warnings.