No idea what a Kawasaki 250r is, no mention of it in the ATV parts list for 2005. So, assuming it has a slide carb, the engine needs air to rev up, either it gets it from past the slide or you have an air leak in the manifold. Check the slide is hitting the adjustable stop, there needs to be a bit of play left in the cable when this happens, to allow for adjustment and the cable can shorten when the steering is turned. If the slide is hitting the stop but it is revving too fast, adjust the stop to make the slide go further down. Check all O rings or gaskets are present on the manifold.

 

2cycle?