HELP Bike is Bogging

[JW]

This is in regards to a friends 92 KX 250. Bike has had a recent top end rebuild at begining of the season. Bike is properly jetted or as close as can be for Colorado riding (Rampart Range)
What is happening is the bike runs fine when it is still warming up but once the bike is warmed up it starts bogging out and stalling. What we found worked to get us out of the mountains was applying the choke. Doing this seemed to bypass the problem. Now he has torn down the carb today and cleaned it and all but it still has the problem. He aslo checked the power valve to see if it was sticking and it is not.
I am going to check the reeds tomorrow to see if they are wore out. I am hoping someone may have had a similiar problem and knows what to do or can at least point us in the right direction. If you have any suggestions of what to do please let me know.
Thanks all, J

 

[David Trustrum]

Yeah I was going to suggest the reeds, but an airleak also sounds likely which is why the choke helps. Pull the ignition side cover & look for oil. Kawaski’s like to leak through the indents surrounding the crankcase seal, or possibly the seal itself. The reed gaskets & manifold/ carb connections bear examination.

While you are at it measure or change the sparkplug cap.