I've had my stock carb empty for a few weeks (did it a quick cleanup and jet upgrades) and now the float bowl won't fill. Petcock is manual and fuel is running from the hose, but the screw in the bottom of the float bowl only gives out a droplet or two.

Any suggestions what might be wrong?

Agreed.  You can also blow a little air through the hose if needed.

Tapping didn't had any effect, but I removed the float bowl and played with the floats a bit -> flooding. Took it apart and put it together again, the bike started straight away without any fuel on the floor.

First start after a bore job and new 95mm piston. I'm happy  

Thanks everyone!