Honda CRF 70 is cutting out?

Just bought a used 2006 Honda 70 (the price was right!) and it's cutting out or bogging down at high RPMs. What is causing this and how do I fix it? I don't how well it's been maintained, but probably not well... the chain was dry and air cleaner needs cleaning, not to mention the bike needs cleaning in general- all bad signs. I'm hoping it's just a carb adjustment or something simple.