My YFZ450 revs up to a high idle when hot

I bought this for my hubby, it is a 2003 YFZ 450. He raced, rides, I am learning. The problem is it will rev up to high RPMs on it's own when it gets hot. Scares me to death, but the hubby just says to go faster. lol. Not much fun for me.

Here is the troubleshooting we have done:
Throttle cable is questionable; checked needle clip position; sprayed the carb to make sure there were no leaks; my dh took the carb apart and cleaned it.

Runs great otherwise, but please don't tell me to stay off the quad, 'cause I may not let him on my boat.

Any insight would be appreciated. It does not look like an air leak at this point, and yes, I know what that means. I might be a girl, but I buy good, if not perfect gifts.  

Thank you for any input or options.

Nance  

check for trash or water in the carburetor, also check the fuel/air mixture, if it is too lean it will rev high.

if the fuel/air mixture isn't right it may load up on fuel one minute and starve the next.  
Keep it RUBBER SIDE DOWN!!