I have a 2008 Beta Rev 3 250 2 stroke.  Stalls going downhill in steep sections.  What carb mod/kit do I need to fix this problem?  It runs great otherwise.  It has a Kehin carb PWK 28mm according to a trials shop here in the USA.  I'm new to trials central - appreciate any help/advice you all may have!  Thanks.

 You need to make sure what carb you run. We also need to know what elevation you ride at. Colorado`s a big state! Most likely it is a float level/idle speed problem.

you can check the float level, but to some extent this is a normal limitation of a carburetor and simply requires some clearing revs to prevent stalling or perhaps  a bit higher idle