I just bought a trx 300ex and i noticed when i start to top off in whatever gear im in it sounds like it's skipping and not firing right. It does the same thing if i simply hold the clutch in and throttle it all the way. I replaced the spark plug and it didnt help. The choke isnt on and its a four stroke so i know its not an issue with the fuel mixture. What should I check next and am I overlooking something?