1974 Yamaha DT250A / DT360A Factory Service Manual FSM (P/N 450-28197-20)

• Coverage and document identity: Yamaha factory service manual for the DT250A and DT360A, identified on the cover by document number 450-28197-20 and supplied as a 207-page PDF. Machine-identification data lists starting frame/engine serials 450-000101 and 446-000101, respectively.
• Factory corrections: Opens with a Yamaha errata section covering model dimensions, ignition timing, clutch and kick-starter clearances, carburetor float data, charging output, and wiring notes, so corrected values can be checked before repair.
• Engine repair: Covers engine removal and installation, cylinder-head and cylinder service, piston-pin, piston-ring, and piston inspection, the DT360A decompression device, crankcase separation and assembly, crankshaft inspection and alignment, and bearing and oil-seal replacement.
• Fuel and lubrication systems: Details the Mikuni VM28SS and VM30SS carburetors, reed-valve inspection, circuit and float-setting work, plus Autolube adjustment, bleeding, output testing, troubleshooting, and external pump service.
• Clutch and transmission work: Follows the wet multi-disc clutch, primary drive, kick starter and tachometer drive, shift mechanism, and constant-mesh five-speed transmission through disassembly, inspection, adjustment, shaft-spacing checks, and reassembly.
• Electrical diagnosis: Separates the DT250A breaker-point flywheel-magneto ignition from the DT360A CDI system, with timing procedures, spark-gap and component tests, charging and lighting checks, switch-continuity tables, and model-specific schematics.
• Chassis and reference coverage: Includes maintenance and lubrication schedules and repair procedures for the drum-brake wheels, bearings, rims and spokes, chain final drive, forks, steering head, rear suspension, swingarm, and controls, followed by troubleshooting charts, torque tables, capacities, clearances, and wear limits.

What Changes Between the DT250A and DT360A

Across nine chapters, this factory shop manual covers the mechanical and electrical repair work needed to maintain or rebuild either A-series model. Yamaha's foreword addresses mechanics and frames the material around disassembly, reassembly, maintenance, and troubleshooting. Its procedures assume the ability to use dial indicators, micrometers, electrical testers, presses, and engine-service fixtures, making it best suited to mechanics and experienced owners with comparable two-stroke repair skills and equipment.

Both motorcycles use single-cylinder, seven-port Torque Induction two-strokes and five-speed gearboxes, but the manual preserves the differences that matter when selecting parts or test methods. The DT250A uses a 246 cc type 450 engine, VM28SS carburetor, contact-breaker ignition, seven clutch friction plates, and its own primary and final-drive ratios. The DT360A uses a 351 cc type 446 engine, VM30SS carburetor, CDI with charge and pulser coils and automatic advance, eight friction plates, and a kick-operated decompression system. Separate factory tables give the applicable port dimensions, jetting, ignition settings, charging output, gearing, oil capacities, and service limits.

Engine, Autolube, and Drivetrain Repair Depth

The engine chapters cover more than a top-end refresh. Procedures extend through crankcase splitting and airtight resealing, crankshaft runout and connecting-rod inspection, transmission gear-dog, shift-fork, and shift-cam checks, shaft spacing and shimming, and final assembly.

Timing and electrical tool callouts identify Point Checker 90890-03031, Dial Gauge Assembly 90890-03002 with stand 90890-01039, Electro Tester 90890-03021, and Pocket Tester 90890-03029. Internal engine work adds the Press Box 90890-02202, Crankshaft Jig 90890-02300, and other case and crankshaft fixtures.

Autolube coverage explains cable and pump-stroke adjustment, line bleeding, output measurement, check-ball and delivery-line diagnosis, and the pump's factory-sealed service limit. The carburetion chapter traces idle, pilot, slide, needle, and main-jet effects; covers removal, cleaning, inspection, float-arm setting, and refitting; and adds reed-petal sealing and installation checks.

Electrical, Chassis, and Diagram Coverage

Common electrical chapters cover the 6-volt battery, rectifier, charging and lighting coils, horn, stoplight, turn signals, lamps, and switch circuits. Model-specific testing then covers DT250A points, condenser, source coil, and ignition coil, followed by DT360A charge coil, pulser, ignition coil, and CDI terminal checks. Diagram coverage includes the complete DT250A and DT360A wiring diagrams, the DT360A CDI operating schematic, charging and lighting connection diagrams, Autolube pump and clutch exploded views, and multi-view cable-routing drawings.

Chassis procedures address front and rear wheel removal, drum-brake and wheel-bearing inspection, rim runout and spoke checks, drive-chain and sprocket service, fork oil and seal work, steering-head races and loose ball bearings, rear-shock oil service, swingarm bushing checks, and throttle and cable-junction maintenance.

During crankcase repair, the halves must not be forced or struck on their mating faces; damage there can compromise the airtight seal required by the two-stroke. After Autolube pump or cable work, correct worm-drive engagement, routing, adjustment, and bleeding are critical because interrupted oil delivery can damage the engine.

Fuel-system repair, ignition testing, brake service, and complete engine disassembly should be performed with the motorcycle secured and with Yamaha's model-specific safety notes and specifications followed before the machine is returned to service.