[ryoasakura]

with my obselete battery i just achieve 13.4v with no headlamp and drop to 8v when crancking (and its hard to get started).

what must i checked urgently , stator / regulator / battery ?

 

[BillV]

You need to check all of that as well as all the associated electrical connections. See the Electrical System chapter of service manual, though there are other things beyond that. You'll need a decent multimeter.