I have an 89 KX250 that runs great for 10-15mins. from a cold start. As soon as it gets warmed up it quits. NO SPARK! Let it cool down for an hour or 2, the spark comes back and runs great for the same length of time then shuts down again???? Any idea what this problem might be????
It's most likely your stator. The stock stators on those bikes didn't last long. I rode a '89 KX250 for several years after going through 2 new stock stators in less than a year I sent the stator to Statorcorp and had them rebuild it - end of problems. I'm not sure if they're still in business or not, but I'm sure there are others that can do it.
My '89 would act exactly as you describe when the stator went out