| Component | Specification |
|---|---|
| Production Years | 1977-1981 |
| Transmission Oil | SAE 10W-30 or 10W-40 (API SE) (0.8 L) |
| Tires (Front/Rear) | F: 3.00-21 (4PR) / R: 4.00-18 (4PR) |
| Tire Pressure | F: 1.25 bar (18 psi) / R: 1.50 bar (21 psi) |
| Battery Type | 6N6-3B (6V System) |
| Spark Plug | NGK B8ES |
The Engineering Milestone: The Cantilever Revolution
The Yamaha DT 250 MX (internal code 1R7) represents a pivotal moment in dual-sport history. While its predecessors relied on traditional twin-shock configurations, the 1R7 was the first production DT to incorporate the “Cantilever” Monoshock rear suspension. This technology was directly descended from Hakan Andersson’s 1973 World Championship-winning YZ250, effectively bringing factory-level motocross geometry to the street-legal enthusiast.
Technical Analysis: Engine and Chassis
The heart of the 1R7 is its 246cc single-cylinder, two-stroke engine. Unlike the peaky nature of pure motocross bikes, the DT 250 MX was tuned for a broader torque curve, utilizing Yamaha’s 7-port induction and Torque Induction reed valve system. This made it exceptionally capable in technical off-road sections while maintaining enough civil behavior for urban commuting.
The chassis featured a high-tensile steel semi-double-cradle frame, designed to accommodate the long-travel Monoshock that ran under the fuel tank. This centralized mass significantly improved handling over the previous DT series, though it added complexity to the frame geometry.
Maintenance Guide for the 1R7 Professional
Lubrication System
The DT 250 MX utilizes the Autolube system, but the transmission requires manual oil management. Use 0.8 liters of SAE 10W-30 or 10W-40. Ensure the oil meets API SE standards to prevent clutch slippage. Change the oil every 3,000 km or after heavy off-road sessions to prevent metal shaving accumulation in the gearbox.
Ignition and Electrical
The 1R7 operates on a 6V electrical system powered by a 6N6-3B battery. For optimal combustion, the NGK B8ES spark plug should be gapped to 0.7-0.8mm. Given the age of these units, checking the ignition timing via the flywheel marks is critical for preventing piston crown melt-down, a common issue in poorly tuned two-strokes.
Wheel and Tire Configuration
For authentic handling, the 21-inch front and 18-inch rear configuration must be maintained. Running 1.25 bar (18 psi) in the front and 1.50 bar (21 psi) in the rear provides the necessary footprint for mixed-terrain stability. If riding exclusively off-road, pressures can be dropped slightly, but rim locks are highly recommended.
Frequently Asked Questions
What is the fuel-to-oil ratio if the Autolube is bypassed?
If the Autolube pump is disabled, a pre-mix ratio of 20:1 to 32:1 is recommended depending on the quality of synthetic oil used.
Why is the 1R7 designation important?
The 1R7 code identifies the specific European and international MX variant which featured the updated frame and Monoshock, distinguishing it from the earlier twin-shock ‘D’ and ‘E’ models.