1978 Yamaha DT400 Service and Repair Manual

Frame code DT400E, part number LIT-11616-01-44. That book is the factory service manual for the 1978 DT400, and Yamaha used it for the 1977 season and for the DT250 as well.

Owners of both bikes get some value from that arrangement, because a single reference covers the pair with the differences spelled out rather than glossed over.

Inside, the engine chapters give disassembly and assembly in working order with the wear limits that matter during a rebuild. Torque figures are specified. Ignition timing and carburettor settings are the factory ones. The chassis sections deal with forks, shocks, brakes, wheels and controls, and the electrical chapter pairs the wiring diagram with resistance and continuity values for testing.

Diagnosis is organised by fault, so a problem can be traced in a set order instead of by swapping parts until something changes.