that's a tough call...I'm guessing either electrical or carb
when you say "half", do you mean throttle position or engine speed?
if it's engine speed, it might be electrical - I've seen it happen with an XR250 where he had a loose ground...you would just have to chase all the electrical to where it attaches on the frame and clean/re-attach the connections
if it's throttle position, my *guess* is that it's carb - low throttle response means the pilot circuit works fine, but higher throttle position either means the bike is starving for gas or there is an issue with the slide or main jet/needle
since the carb is easier to figure out, I would try that first