hi, i just went through my mikuni on my 91 lt250r and set the mix screw at two turns out from the lightly seated position, per the book. i fouled the plug after a couple hrs ride time and its running a bit rich, is it counterclockwise to lean it or ritchen it, book dosent say? thanks.

 

Ok, a few things here.

The idle mixture screw is just that; it controls idle mixture and not too much after that RPM. It has nothing to do with total fuel flow at speed; that has to do with main jets.

If you fouled plugs after a few hours ride you are way way way way too rich on your main jets. (If the plugs were fuel fouled and not oil fouled)

Take some time and read this artical:
http://www.motorcyclecarbs.com/carbs101.pdf

It's a great write up; you will like it because it answers your questions.