i have a 2003 250 recon that i just rebuilt the top end on. before the rebuild it smoked thus the rebuild but it had plenty of power up to the smoke and even while it smoked it had power.i had a double snorkel on it and it had alot of 90 degree fitings on it but it still ran fine i had nocked the baffles out the muffler but still ran fine.after the rbuild it ran fine for a few days then one morning i got on it and it cranked right up and i putted back to a feild turkey huntin when i came home i went to to go fast and it started missing about half throttle i cleaned the carb put it back one and still did it i changed the 1.2" pvc snorkel kit to a 1.5"pvc with no 90s in it and it still ran like this it would just bog like it wasnt getting gas it would blow air back at me out the snorkel i ran the needle jet out prolly 5 to six turns and it started running better but will still miss if i try to load it up my ramps or put a good load on it does anyone have a clue to what could b causing this to happen.

 

Have you tried running atv with the air cleaner lid off of box to see if the pipes are to small. if pipes are to small and thats the problem its should run better with the lid off. just do this for an test to see if gets better or worst or no change and then update us..