I had similar problem with my two lt250r, i fix the problem replacing the needle jet ( the hole )  it was worn out , and at the same time i replaces the jet needle ( needle ) .

a worn out needle jet that cannot be see with eye. created a rich condition that can be compensated with idle air screw at idle  . but the rich condition occur until 1/4 throttle.  that i cannot compensated with the needle.

Carl, Irecacer, Carl point me to this problem and he was right.