gepe wrote:I decided that at 47000 km it would probably be a good idea to decarbonise the expansion chambers with the caustic soda method. Well, in the end result on the first chamber was satisfying but now there is water trapped somewhere in the chamber! No matter how I turn the chamber, it just doesn't come out.
I can remember that I have seen pictures of NSR250 expansion chambers cut in half and that they chambers have two concentrical shells with some space between them. So I have drilled two 1 mm holes in the outer shell in the hope that I would hit the cavity but of course I wasn't successful. There must be more than one cavity....

The picture I remember of standard MC21 chambers, showed they had a second 'jacket' welded over them. Possibly with some kind of sound deadening material in between too.
Anyway, the jacket was removed, which lightened the pipes.
Was it Bill Limb, or Lamb who did it? Perhaps Andy or Dave Ett can tell us.
If you have got water trapped in this jacket, the best way to get them dry is to get hot air/exhaust gasses blowing through the pipes.
Failing that, store them in the warmest place/room you can find, until you can run them on the bike again.
Better still, get some Tyga stainless exhausts, and leave the originals in storage for when/if you sell the bike.