What do you mean if it's not efi. My machine is a 550 h1 efi trv
sorry I had a brain fart there??
another thing I have seen is when folks run a lot of ethanol fuel and atv sits a lot, this happens to a lot of atv's (all brands) at my hunting camp, where 10-12 atv's site 5-7 months out of the yr
all the guys running ethanol , there atv's backfire often, guys that ONLY use ethanol or run atv often, don;t seem to have issue's
so, I also thing its a fuel issue
from either just the fuel running, or how ethanol fuel builds crap varnish like coatings on things over time
so, could be a fuel flow issue's too due to something being some what blocked up ,
maybe try running some sea foam or like treatment(not than magic in a can fixing all things)
but it might help some here if its mild issue and can clean/free things up for you?
before going deeper into things!