1987 Yamaha YZ125 JYA2HG-Series Service Manual (P/N VIN JYA2HG000HA000101)

This original OEM publication serves as the foundational technical reference for the liquid-cooled 2-stroke engine and semi-double cradle frame of the 1987 YZ125. Designed specifically for shop-floor efficiency, it delivers the precise specifications mechanics and DIY builders need for complete race-readiness.

• 47 exploded-view diagrams and 15 service torque tables streamline complex teardowns and complete rebuilds.

Component Maintenance & Teardown Coverage

Organized into seven core chapters, this documentation details every critical area of the motorcycle. It provides comprehensive technical guidance for carburetor tuning, reed valve inspection, and exhaust system servicing. For deeper mechanical work, the manual outlines extensive diagnostic and measurement data for the cylinder head, piston assembly, and the YPVS governor.

The transmission and shift shaft sections include crucial torque values for internal fasteners and bearing clearances. Working on liquid-cooled 2-stroke engines involves high-pressure components and rapidly moving parts. Always cross-reference these precise factory specifications during assembly to ensure safe, reliable performance on the track.

Critical Specifications and Reference Data

This document acts strictly as a comprehensive data and specification reference outlining factory parameters, rather than a basic step-by-step repair tutorial. Essential system specifications include:

• Bore and Stroke: 56 x 50 mm
• Compression Ratio: 8.4 to 10.5 : 1
• Cylinder Head Bolt Torque: 30 Nm (21 ft-lb) using a cross-recess sequence
• Spark Plug Gap: 0.5–0.6 mm
• Standard Ignition Timing: 1.06 to 1.36 mm before top dead center
• Clutch Plate Wear Limits: 2.7 mm minimum thickness, with a maximum warping tolerance of 0.05 mm