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

Yamaha factory service manual for the 1974 DT250A and DT360A, P/N 450-28197-20. Nine chapters cover maintenance, troubleshooting, disassembly and reassembly.

Bookmarks, Diagram Detail and PDF Format

Search and diagram viewing: Embedded PDF bookmarks cover the chapters, appendices, wiring diagrams and cable routing. Manual text is not searchable. Small schematic labels require zooming; each model's full wiring diagram is split across two pages.

One printable PDF with 207 pages, including the covers, blank pages, two factory errata pages and two additional electrical-reference pages.

Two-Stroke Engine, Autolube and Chassis Service

The selected topics below highlight part of the coverage. Other subjects include muffler/spark-arrester maintenance and tire-and-tube replacement.

• Factory corrections: Errata cover 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: Step-by-step procedures cover removal and installation, cylinder-head and cylinder service, piston-pin, ring and piston inspection, crankcase disassembly, crankshaft alignment, bearings and oil seals.

• Fuel and Autolube: Carburetor removal, cleaning and float adjustment, reed-valve inspection, plus oil-pump adjustment, bleeding, output measurement, troubleshooting and external pump service.

• Clutch and transmission: Disassembly, inspection and reassembly cover the wet multi-disc clutch, primary drive, kick starter, tachometer drive, shift mechanism and transmission shaft spacing.

• Electrical diagnosis: Model-specific timing procedures, spark-gap and component tests, charging and lighting checks, switch-continuity tables and wiring diagrams.

• Chassis and service references: Maintenance and lubrication schedules accompany drum-brake, wheel, suspension, steering, swingarm and control service, with troubleshooting charts, torque tables, capacities, clearances and wear limits.

Its procedures are best suited to mechanics and experienced owners with comparable two-stroke repair skills and the required measuring equipment and engine-service fixtures.

What Changes Between the DT250A and DT360A

The DT250A pairs a type 450 engine with a Mikuni VM28SS carburetor, contact-breaker ignition and seven clutch friction plates. The DT360A uses a type 446 engine, Mikuni VM30SS, CDI with charge and pulser coils and automatic advance, eight friction plates, and a kick-operated decompression system.

Both use the seven-port Torque Induction layout, with separate port-dimension, jetting, ignition, charging-output, gearing, oil-capacity and service-limit tables. These model-specific references help select matching parts and test methods before repair.

Engine and Drivetrain Repair Depth

Procedures extend beyond top-end work to airtight crankcase resealing, crankshaft runout and connecting-rod inspection, transmission gear-dog, shift-fork and shift-cam checks, shaft shimming, and final assembly.

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.

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.

Carburetor Circuits and Autolube Service Limits

The carburetion chapter relates idle, pilot, slide, needle and main-jet operation to throttle position. Float-arm setting, refitting, reed-petal sealing and installation checks are described.

Autolube troubleshooting covers check balls, delivery lines, seals and air leaks. The pump is factory-sealed: disassembly is limited to the illustrated components; a pump that fails output specifications is replaced.

After pump or cable work, correct worm-drive engagement, routing, adjustment and bleeding are critical because interrupted oil delivery can damage the engine.

Electrical Tests and Wiring References

Common electrical coverage addresses the 6-volt battery, rectifier, charging and lighting coils, horn, stoplight, turn signals, lamps and switch circuits.

DT250A sections explain points, condenser and ignition source-coil operation, with ignition-coil and condenser tests. DT360A diagnosis adds charge-coil, pulser, ignition-coil and CDI terminal checks.

Diagram coverage includes 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.

Wheel, Suspension and Control Service

Chassis procedures address wheel-bearing replacement, 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.

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.

Piston-Fit and Autolube Output Examples

For both models, the piston inspection section specifies 0.040–0.045 mm piston-to-cylinder clearance and a maximum wear limit of 0.1 mm. The measurement discussion distinguishes the specified fit from the replacement threshold.

Autolube output specifications for both models are 8.8–9.7 cc per 200 pump strokes at maximum throttle and 0.95–1.2 cc per 200 strokes at minimum throttle, supporting the manual's pump-output assessment.