Good question! You need to give a little more info. It is hard to give good advise, not knowing the sled history. If it was running good and you haven't made any carb setting changes, you may just need to remove the float bowls and remove the small pilots. You can soak them in carb cleaner, and blow them out with air.
I suspect the temp sensor, if it is bad it will retard the time and do that. you can get a new one for around $20. If it doesn't fix it its good to have one as a spare anyway. probably the best place to start and the most common. If you have the carbs out go through and clean them. and check the reeds while you are there
i recently bought an 03 800 159 edge for a back up and it immediately had these exact symptoms. thinking it was dirty carbs cause it was supposedly sitting since last spring i started there. took it out and same symptoms, took the carbs back off and dismantled the slides cause one had lost connection with the shaft. found a whole bunch more missing stuff in the top of both carbs that link the slide to the shaft. bought all of that stuff and still wouldnt run on the hill but the can wallered out the dougnut to the pipe at the same time. fixed the pipe and started looking at the needles and they were way off and the air screws werent even there?? so i bought the correct main needles and air screws just got them the other day and it looks as though the last guy drilled out the threads that hold the air screws in the bell of the carb?? noe i need to order new bells too!! what a crappy time its been with this thing
my point is if the sled came from the flat land which i assume mine did and needed way more air maby look into the hole where the air screw goes and make sure they havent drilled it out?? but who knows if thats even the problem with mine??