I have an 84 XR200R thats doesn't like to run after a few hours of riding. It will start and idle but when I release the clutch in gear it stalls even if I give it lots of gas. The first time it did it I cleaned the carbs and the second time I cleaned the tank/lines/carbs. Now it did it again yesterday. It runs great until about the 3 or 4 hour mark. Any ideas what could be causing this?