Welcome. The manuals are available here for you to read and download. If you intend on really riding the bike on the road safely and have great reliability, and the ability to do roadside diagnosis and repair, then you must read the manuals and study your bike. I recommend buying the physical book so you can handle it when your hands are dirty, and its much easier having all the info right there instead of going back and forth with the phone.
Most years of the XS400/360 are interchangeable as far as the manual and parts go. The big difference between years would be the carbs, but carbs all share the same basic design features anyways. Carbs have very small parts and soft brass components, use the right tools.
Its probably a clogged jet inside the carbs, and maintenance.
If you recently purchased this bike or your uncle finally gave it to you, whatever the case may be, you must inspect the bike 100% and it would be very wise to perform all of the maintenance requirements. Youll need a few metric tools and screwdrivers, a manometer, and a voltmeter. For me, I need an induction timing light. You should have a timing light too, but you have electronic ignition and it shouldnt ever really need adjustment.
Start with the engine, I would probably start with a compression test if you know nothing about the bike.
Youll want to clean the carbs and sync them and set the valve clearances. Make sure you have a good battery, Id probably buy a new one if its more than 2 years old. Get it running and do a basic voltage check to be sure you get good charging power and idle voltage. After this, Start it and enjoy.
This is an older machine and their owners really have to know what theyre doing to enjoy the motorcycle and keep it on the road. You dont take this bike to the dealer to be worked on. Maintenance intervals may be more frequent than modern bikes but its not too demanding, even for me having a points ignition. I enjoy my old machines and the routine maintenance that comes with it.