I also am having similar problem. I have been riding my quad for a couple of seasons with no issues. Then, starting a couple months ago, at higher RPMs, the motor will start sputtering. If I lay off the throttle for a second, then get back on it, then I get normal response. It will actually run almost normal until the quad comes up to operating temps. Once I am at operating temps, then the sputtering becomes more apparent. I've had myself convinced several times that I was just not getting fuel - it feels the same if running out of gas. I have cleaned the carb a few times and replaced the plug. The plug looked normal - light tan. Still, I can't seem to find the root of the problem. Since I have been running well for a season and a half, I am sure that the issue is not jetting. Other than valves, what else can I check?