i havent got the base manual so cant tell you where the screws are , they are going to either be on the side of the carb or underneath
I found this useful guide
http://ninja250.kingston.net/~jeb/Kawasaki Ninja 250 FAQ.pdf
but it doesnt mention the screws
either way
do not unscrew your main jets
you need to find the screw
settings
mix screw is 1 3/4 turns out
float height is 17mm +/- 2 mm
if you bike has a fair few kays on it they do get bad hot idle due to excessive engine gas blowby being pumped back into the airbox via the crankcase breather
solution is to remove the hose from the airbox and put a little filter in it and possibly into a catch can as it can blow some oil out of this hole
block off the hole in the airbox and now your engine will thank you by not having hot oily air pumped into it