1981 Yamaha DT50MX, DT80MX Service Manual

Yamaha's first-edition service manual, published in March 1981, covers the 1981 DT50MX and DT80MX with air-cooled, single-cylinder two-stroke engines and Yamaha Autolube lubrication.

• Engine repair: Piston and cylinder inspection, crankcase disassembly, crankshaft checks and installation, plus cylinder-head, piston and exhaust decarbonization.

• Clutch and transmission: Wet multi-plate clutch inspection, clutch housing and boss wear checks, four- and five-speed transmission layouts, and shift-mechanism adjustment.

• Fuel and oil injection: Teikei carburetor tuning, float and needle-valve inspection, reed valve checks, Autolube pump adjustment, and oil-system bleeding.

• Forks, wheels and brakes: Front fork disassembly and seal service, steering-head assembly details, drum brake maintenance, wheel runout checks, and chain inspection.

• Electrical diagnosis: Contact-breaker ignition checks, flywheel magneto diagnostics, charging-current measurements, rectifier testing, and lighting-system fault tracing.

• Workshop reference: Market-specific wiring diagrams, cable and pipe routing, exploded-view assembly diagrams, tightening torques, fluid capacities, and component wear limits.

Written for Yamaha dealers and their qualified mechanics, this manual assumes familiarity with the machines' basic mechanical construction and maintenance procedures.

Model Identification and Fuel-System Variants

The identification chart links the DT80MX to France (5J0), Germany (5J1) and the UK (5J2), and the DT50MX to the Netherlands (5M3), Belgium (5M4) and the UK (5M6). Engine and frame starting numbers are provided for these model codes.

The DT80MX uses a Teikei Y16P carburetor. DT50MX specifications distinguish the Y12P for the Netherlands and Belgium from the Y14P for the UK. This helps you select the applicable carburetor settings before diagnosing poor running.

Autolube coverage includes cable alignment, minimum pump-stroke inspection and adjustment, and the circumstances that require bleeding the pump and delivery lines. Reed valve inspection covers stopper height, seating clearance and assembly orientation.

Engine, Clutch and Transmission Repair

For top-end repair, the manual provides piston-clearance, ring-end-gap and cylinder-wear specifications, with piston and ring installation illustrations. Bottom-end coverage adds connecting-rod clearance checks, crankshaft runout inspection and crankcase sealing notes.

The Crankcase Disassembly illustrations identify the required pullers, including crankcase separating tool 90890-01135. Crankshaft installation drawings identify the setting tools, while the Clutch and Transmission exploded views distinguish component arrangements and show lubrication and tightening points.

Clutch service covers friction-plate wear, plate warpage, and grooving of the clutch housing and boss. The shifter section explains the relationship between shift-lever and shift-drum alignment when gear changes are not smooth.

Monocross Suspension, Forks and Brakes

The Yamaha Monocross section explains the De Carbon system, floating MXS valve, free piston and nitrogen gas chamber. Coverage includes operating principles, handling and disposal precautions; internal shock-absorber rebuilding is outside its scope.

Front Fork and Steering Head exploded views accompany fork oil replacement and seal installation information. The fork-seal instructions warn that installation tools can damage the inner tube.

Brake coverage goes beyond cable adjustment to shoe wear, drum inspection and correct alignment of the brake wear indicator. Wheel and final-drive sections add spoke adjustment, rim runout, chain stretch and sprocket wear checks.

The nitrogen-charged shock requires special handling. If a repair procedure exceeds your current workshop setup, consult a certified motorcycle technician.

Electrical and Diagnostic Specifications

The ignition troubleshooting charts work through spark production, breaker points, the condenser, ignition-coil resistance and flywheel-magneto coil checks. Charging-system diagnosis covers battery specific gravity, the fuse, charging current, rectifier condition and harness continuity.

Circuit drawings identify regulator connections, and the test data distinguish the German DT80MX charging and lighting configuration. Six wiring diagrams cover the three DT50MX markets and three DT80MX markets, with component labels and wire-color codes. Cable Routing Diagram pages also show control cables, oil lines and battery breather routing.

PDF Navigation and Diagram Readability

Supplied as one printable 135-page PDF in a ZIP, with English, French and German technical text; the total includes two opening pages and eight wiring-diagram and legend pages. PDF text search is unavailable, and four generic bookmarks jump to document sections rather than individual chapters. Small wiring labels require zooming.