time an 04 250ex

Pull the round plug out on the flywheel side of the motor,pull the the top cover off to expose the cam sprocket and chain,look through the round hole at the flywheel and line up the mark next to the T on the flywheel with the index mark in the top of the round hole this is your timing mark, make sure the dot on the camchain sprocket lines up with the index mark just above the sprocket but be sure the timing mark is also lined up if not re move the camchain sprocket bolts rotate the motor until the timing mark is lined up and move the sprocket in the chain until the cam sprocket marks line up put bolts back in sprocket and timing is set. Hope this helps,

i was letting it idle and it just died, now i have to give it full throttle and it backfires, do you have any suggestions?

Back fire through carb or the exhaust? Sounds like maybe a bent valve.