Someone's off their meds again..? Over 3 turns on the fuel screw calls for the next bigger size pilot jet.. You won't have to worry bout that tho - a #40 will be right on..

I'm thinking that's right too, it would be richer on the carbs with the adjustment on the intake side of the slide, but I also agree the odds are the Mikuni 40 will be right, since the idle jetting is just plain lean from the factory and usually most all the bikes take the same one since there is adjustment that no other circuit has so altitude is mostly able to be dealt with in most cases.

Woops, had that backwards, sorry.

Also... you may want to just get a Kouba extended fuel/air screw for easy adjustment.

OK, just to clarify (because I'm left-handed, mildly dyslexic and accustomed to working on two-strokes:D) CLOSING the screw on the OEM '09 KLX carb RICHENS the mixture? OPENING the screw LEANS it out?

I don't think so.

The screw controls fuel flow through the pilot jet system. Closing (turning in) lessens fuel flow and therefore is LEAN. Opening fuel screw provides more fuel for RICH.

Is that opposite of your two-stroke?

No, Anciano. The screw controls fuel flow from the pilot system into the engine. Screwing it out increases fuel flow to the engine, up to the point that you fully uncover the port - Screw it in and you do the opposite, up to the point that you completely close off the port. The pilot jet along with the pilot air jet emulsifies air and fuel before it goes out the port, and at that point, past the screw.

OK, got it. Now if I can just remember that "lefty-loosey, righty-tighty" thing.:D

Imagine how confusing it was for me when I had both an RD350 and a DR350. And because I once had a Honda XL250 I keep calling this bike a KXL250.

I don't think it really matters whether it is making it leaner or richer, forget about that.
What you need to do is tune it, if it is too small (3 turns+) you need a bigger jet, if it is to big (less than .5 turns) you need a smaller jet. That is all that matters.

There's a little more to it than that. I live at around 2200 feet but most of the time head on up to trails and forest roads at anywhere from 5000' to 7000'. I would like to be able to adjust "on the fly" as altitude changes. Of course the fuel/air screw is a limited way to compensate for altitude but it's all I've got. I really wanted a fuel-injected bike for this reason, but a suitable Yamaha WR250 never turned up on Craigslist.