Start with a full tank. Run it down to 1/4 when it starts to heat up, fill the tank back to full. Don't stop the motor durring this process. If the temp goes back down, you know the problem is in the fuel delivery. If it stays hot, you have either a tuning issue, or an air leak in your carb or back plate. I had a similar problem a while back and found that I had gradually "tuned" the carb for a rich bottom and a lean top. My track is small, so the problem didn't really show itself until the end of the race. I reset the needles and the problem dissapeared. Now I have a spare tank and new fuel line.

I am not a newbe and thought I was pretty good at tuning, but I had made a mess of the tuning.