Hi GenX - make sure the scaling rail extends all the way to the very end of the other curve, and in fact with a scaling rail it's not a bad idea to let the scaling rail go a ways past the other curve to make sure it fully covers all areas of it.

You can get that kind of little spike at the end when there is an area that the scaling rail did not cover, basically the sweep pops up to full size if it could not intersect the scaling rail at any one spot.

You may find using 2 rails for such a shape (select both curves as rails instead of using one as a scaling rail) make work better for your case there. When using 2 rails rather than 1 rail + scaling rail you would want to make sure both rails come together to touch at a single common point, you don't want one the end of one curve to be unaligned to the other curve's end.

Check out these previous posts for some more description and related discussion of scaling rails:

http://moi3d.com/forum/index.php?webtag=MOI&msg=4434.2
http://moi3d.com/forum/index.php?webtag=MOI&msg=1807.7
http://moi3d.com/forum/index.php?webtag=MOI&msg=4785.7

- Michael