I will say this, take as you wish?
up my hunting camp, we have a fleet of atv's. many sit for weeks and or months, some guys re more tool turner's than others.
some guys make the extra effort to run ONLY non ethanol fuel in there atv's and MOST don't
ALL the guys that run ethanol fuel , get occasional or more often backfires?
those that run ONLY NON ethanol fuel, NEVER have them?
so from MY experience the fuel you use itself can cause things to backfire, even when carb and all other things are GOOD!
not always possible to get ethanol fuel , but might be worth a try to run some and see , if you have access to it??