I've had my 600 for about a year and half allready and have had this annoying problem. When full throttle, once its been used for 20 minutes or so, it sounds like the choke is on partially, but its not. they did the valve adjustments (as per the 100 hour maint) and still the same. I changed the spark plug made it a little better, but back to the way it was. The fuel filter hasn't been changed since new. Could this be the problem?