I am having some problems with stalling with my 2006 KLX250. It will start fine, and rev fine in the garage, but once I get it out and start riding it, it starts to stall when given throttle. It will get so bad that it will not rev above 5K with no load, and sometimes stops running altogether. This will last a bit till it cools and I start it again, or it might go away and I can ride (not far from the garage at present). This problem started when it was bone stock, and I suspected the carburetor was the culprit. I installed a DJ kit (replaced the main jet, drilled the slide, and ran the idle screw out 2 ½ turns, installed the new needle). The slide and floats looked like they were operating good and I cleaned the pilot jet as well. There was no dirt or varnish to speak of. I also de-snorkeled it and put on the Acerbis tank. After putting it all together again, the problem persists.
Any ideas as to the problem or troubleshooting?
Thanks

 

How does the spark plug look?

 

Did you tune the air/fuel screw? or just turn to some arbitrary setting?
Did you take the air lid off?
Or the slide diaphram is a miss.... this is probably the first thing to review.

 

Pinched fuel line restricting flow? Vent tube? If they use a line from the cap and it has a 1 way valve on it, make sure it is inserted the right direction.

 

I would agree with KLX678 but most likely a pinched fuel line. I had a very similar problem. I put a clear plastic fuel filter in line about 3 inches before the carb. This allow you to see the fuel flow.
In my case the problem with the fuel line was fine when cold, but once things warmed up the fuel line softened and would pinch partially off due to a little to tight of a bend radius. The resulting fuel flow allow it to run barely just above idle. The short term fix was to unbolt the petcock let it hang to straighten the fuel line out. New fuel line, clear filter and rerouting fixed it.
Good Luck

Someone on the ADV Rider KLX250 page is describing very similar symptoms.

 

The spark plug looks good; I tuned the mixture screw. The instructions included with the Kouba adjustable screw had me run it out 2 1/2 turns.
I will replace the fuel line with some clear hose to see if that helps.
Thanks!

 

It definitely gives the indication of restricted fuel flow, especially since it's only occurring under load. Starts, idles, and revs fine without a load, but once under way it stumbles and even stalls. While it could also be several other possibilites, I'd start by checking the fuel delivery system...petcock flow and fuel feed line...and also the fuel venting system...tank/cap/carb vent lines and openings.

 

Run it with the gas cap off as the vent may be stopped up.

 

Stalling still occurs with the fuel tank cap off. Replaced all the lines from the tank to the carb with clear lines; fuel looks to be flowing good. The carb vents are clear. I might start to chase the spark. Does anyone know where I can get a known good used CDI igniter box? The price is pretty steep new.

 

Tune the kouba screw!