Hi all,Ihave a 2000 model 250 beta rev3. 
The bike starts and runs at low revs perfectly,at about half throttle it starts to missfire,and can't seem to clear itself. 
I have cleaned the carb which did not help,adjusted the air screw to lean the mixture and it seemed to help a little. 
I could be wrong,but the bike seems to be running very ritch,but have ho idea what has caused it. Could be a weak spark?? 
Hope someone can help,many thanks