Hmm...
Which power pin does that adapter connect to? I do know the nunchucks are supposed to be fed 3.3V and may not like it if you feed in 5V.
Edit: quick search indicates that some chucks don't care, and other return max values which might be what you're experiencing.
From
http://todbot.com/blog/2008/02/18/wiichuck-wii-nunchuck-adapter-available/
Tenshi
June 4th, 2010 at 4:20 pmHello all and tod,
I've got a WARNING for those using a “Black nunchuck”:
It will only run/work on 3.3v power.
If you use it with 5v you'll only get maxed out values however it is not blown up,
run it on 3.3v and it will work like a sunnyday.