My DT400 has an issue with shifting.  I have to pre shift almost everything, meaning I am nudging the shifter a tiny bit inorder for it to be where it needs to be to properly shift.  It feels like maybe a spring or something isn't returning where it needs to be after each shift.  Anybody have a thought on what this may be?  The clutch works good and I still ride it
 

There is an adjustment on the shifter under the clutch cover. You center the prawl over the two pins on the shift drum in second gear.

Thanks, I will look into that.

Missing detent ball and spring is my guess. 

Happens a lot on that model, looks like a drain plug, parts fall out and never get replaced.

I had a similar issue with my DT1 . I had suspected weak return springs on the shifter claw mechanism .
i was wrong . I had a slight bend in the shifter shaft . It was fine changing up gears but needed a nudge to
be able to select gears when changing down . That may not be the case with your bike , but it's worth bearing in mind .

Another possibility is the return spring has been fitted without the ears straightened out, ie--uncrossed--so doesn't go parralel across the two items it needs to sit in line in the resting position between gears so it can pickup the next gear. If that's the case then the spring would only be about half strength for part of the travel but not enough strength to return it fully let along bring it to central again. That's my guess seeing everything else has been covered by others. Just having a look on cmsnl parts it looks like the main stopper #16 isn't adjustable eccentricly like on the old DT1 etc so be sure it's not come loose in the case but #11 the small one 'is' eccentric for centralizing--providing the return spring is fitted correctly.
 

I have made an adjustment on the 10 mm nut on shift lever one.
That appears to be the only adjustment in that. Other than bending the tiny spring behind lever shift 3 part 22.
Which I do not see that spring on the parts diagram.
I will try it out soon and get back.
Thanks for the replies.
 

It still needs to be 'preselected' nudge to downshift, other than that seems ok.
I'll have to go back in there.

And by the above post specifying my issue, I will check that shifter shaft when I get her apart.