Code 1263

[Emile]

Hi, I need some help regarding code 1263 - fuel tank evaporative purge valve. Where is this valve located and what does it mean? Car idle is fine but under powered with a full fuel tank (seems crazy), it seems to improve with less fuel in the tank - Can this have something to do with the preasure in the tank or fuel system somewhere?

thanks
Emile 1996 525i M50 200000km