Hi, Semso and the usual suspects are:
1. Spark plugs in bad condition or partially fouled.
2. Spark plug cables in bad condition and leaking.
3. Spark plug gap too close or too wide.
4. Faulty ignition coil, module, and or sensor.
5. Battery nearly discharged.
6. Check battery terminals for damage or corrosion, check the battery cables at "BOTH" ends for loose, corroded, or broken connectors, "INSIDE" and outside the cable harness, perform connector wiggle test and check cables with an ohmmeter if necessary.
7. Intake air leak.
8. Water, dirt, or rust in the fuel system, carburetor and/or filter.
9. Fuel tank vent system plugged or carburetor vent line closed off.
10. Carburetor controls misadjusted.
11. Damaged carburetor.
12. Loose, dirty, or corroded ignition module connector at crankcase.
13. Faulty MAP, CKP, O2, and/or CMP sensors.
14. Incorrect valve timing.
15. Weak or broken valve springs.
16. Damaged intake or exhaust valve.
For more information about your issue, please visit the websites below. Good luck and have nice a day.
Motorcycle Carburetor Cleaning and Rebuild part 1
Troubleshooting Motorcycle Ignition 1982 Yamaha xj550 maxim http://www.49ccscoot.com/manuals/Keeway_50cc_2T_Service_Manual.pdf
Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/gregg_c0ec1df182c7330e