Definately YZ Yammy but exactly which one is a little hard to tell. Its not the early ones that were strap on because of the mount at the back.
(http://i96.photobucket.com/albums/l187/mx250a/224b_3.jpg)
But other than that it could be from a range of YZ's of the period down to the YZ80. We might be able to work it out if you showed another pixie or gave the length. But as God points out it probably YZ250/360c.
I would give you are fair price if you are interested in selling.