If you need to execute fixes on your own Yamaha YZ125 then it is always a good plan to have a Workshop Manual to use as a reference just in case you want to know tips to take off a particular component much like the Gearbox and install it again properly.
Yamaha YZ125 Workshop Manuals offers clear unique diagnostic facts for your automobile therefore if something is playing up it helps you identify what the situation is and tips on how to make it better. You will be able to assess if you can do this on your own or that the job is too big you could make a booking for your YZ125 into a decent repair garage.
Yamaha YZ125 manual