I have a 2003 660. I leave home and run to my camp about 40 minutes through a trail top speed about 15 km/hr. I leave the camp and get on the fire roads and open it up.It starts to "crackle" not really a backfire and loses power.I have to feather the fuel to keep it going.Eventually it will stop.It will start right up and it will run fine at low speed but does the same thing if I try to open it up. Any idea whats up?