Loose valve stem core. But before bringing your motorcycle to the vulcanizing shop for a patch, check first your valve stem and tighten it's core (if loose) by turning it clockwise. But to complete this solution, you still need to have a valve stem tool to screw the valve core and a portable air pump to inflate your tire.
Water power. You can check air leaks using water. Pour some water directly at the valve stem, if bubbles are visible, then there is an air leak. Observe where the bubbles come from. If it is from the valve stem, you may have a loose or defective valve stem core. But if it comes from the side of the valve stem, then you have a hole in your tire interior.
|If bubbles come from where the green arrow is pointing, valve stem core problem.|
But if at the red arrows, there's a hole in your interior rubber.
Motorcycles don't carry a spare tire. So you need to look for the nearest vulcanizing shop when you have a flat tire (unless, of course you can fix it yourself). Run slow and remove as much load as possible from your motorcycle to prevent more damage to your tire. Upon reaching the shop, you may either ask the vulcanizer to patch the hole in your interior or replace it. It is safer to replace it though, if circumstances permit.
Have a safe drive, everyone. Yahweh bless.