I have 21" Bandits on the front of my KFX. IF i put the handle bars straight, i noticed that the wheels curve inwards in the front. If you stand behind the quad, the curve is so bad! stand in front, it doesn't look as bad,. but its noticable. Anyways, when I am riding, when i hit around 3rd gear and up, the handle bars shake like crazy, and i don't know why.. does anyone here?
I am thinking it is because of the curving in that when i get higher in speed the wheels are messing up when they roll..

please help!

 

It's a 4 stroke?

Oh, sorry... I couldn't resist

The 'curving in' you are mentioning sounds like Toe or Toe-in. It's a simple adjustment to correct, and will make your machine handle much better once corrected.
Check out your service manual for instructions.

 

A little toe is good, too much is bad. Check your shop manual like Dune said, it will tell you how much you should have.