About the only active discussions on this forum revolve around KLX 250S bikes. For future questions, you may have better luck at this forum:

 

Worked in a dealership and saw this numerous times. Odds are the old gas evaporated leaving crud in the float bowls. When fresh gas comes in the crud either floats bits out into the very small pilot jets, or gets gummy and plus them. Characteristic is runs on choke, may idle, but will not take normal smooth throttle. If you keep working the throttle to get the revs up you eventually get to the level where the slide/needle/main jet control throttle. They don't clog easily since they are bigger.openings.

You can have the carbs cleaned. If you can take them off the bike and pull the pilots to see if they are clogged. You may need to tear them down and have them cleaned in a carb washer or ultrasonic cleaner. They can get cleaner in and worked around in areas that are unreachable with spray and a wire. If you do them yourself, look up something about using Pinesol to clean carbs. Here is a link to a thread on it at the ADVrider forum

One possibility before going to the tear down, some riders have done something called Seafoam gas treatment, which may resolve the problem without tear down. Never tried it and don't know for sure.