If you want to do adjustments on your own Derbi Senda XRace 50 R then it is usually a great idea to own a Workshop Manual to keep just for personal reference in case you want to know the right way to take off a certain component for example Gearbox and install it again successfully.
Derbi Senda XRace 50 R Workshop Manuals gives you particular diagnostic wisdom for your car or truck so if something is not working it helps you understand what the situation is and tips to repair it. You can then analyse if you can do this on your own or in case the job is too big you can actually make a reservation for your Senda XRace 50 R in with a trusted mechanic.
Derbi Senda XRace 50 R manual