If you need to do fixes to your Yamaha DT 400 MX then it will always be a good plan to have a Workshop Manual to use as personal reference if perhaps you want to know simple methods to strip off a specific component for instance the Gearbox and install it again in the right way.
Yamaha DT 400 MX Workshop Manuals offers you specific diagnostic information for your car or truck therefore if something is faulty it helps you work out what the situation is and methods to resolve it. You will be able to determine if this can be done by yourself or in the event the job is too big you can still make a reservation for your DT 400 MX in to a decent mechanic.
Yamaha DT 400 MX manual