Try adjusting the valves. Seems to be a real problem on the automatic atvs as the engine RPM is always 'up there' and causes 'em to wear a bit faster. The intake can tighten itself up over a period of time and cause hard starting and erratic or non idling. Also check the carburetor vent hoses to make sure they're clear. Clogged ones will generally idle just fine, but when you throttle up it'll lean out & die. Sounds like tight intake valve to me, but then again what do I know?
"it is better to appear ignorant than to open your mouth and remove all doubt"