I have a 2004.5 Sportsman 600 vTwin with 1050 miles on it. It'll start right up with no problems. After the warm up, I'll give it some throttle and it will bog down to the point of stalling unless I either feather it or release the throttle all together. Seems to be worse when cold. It'll roar to life once I get past the point where it bogs down. I've checked/replaced all fluids, air filter, spark plugs. Any ideas?