yesterday my dad asked me if i could take a look at his 400 today because it was running goofy. well i rode it around the yard and didnt notice anything so i took it on the road and right when it gets right at 5000 rpm it starts missing badly. it was fine 5 days ago when i rode it. so i called the dealer and asked to speak to a mechanic but nobody was there except her. so i tell her and says she thinks its because we use gas with ethanol. which i dont think it is cause we have used ethanol in it for the last 2 years with no probelms. i think it could be:
rev limiter
fuel filter
carb float
dirty main jet
if anybody has an idea as to what it could be that would help alot. thanks

 

update:
it seems that it will only miss in high. it wont in low, reverse, or neutral. so that throws out what i had thought it was earlier. could it be the reverse rev limiter because thats what it sounds like when it does miss.