I have a 2015 Yamaha that had the same problem. I too went to TOV for help. My plea for help was answered.
Go to O'Rielly's auto parts on 301 in Wildwood and buy a bottle of RxP (red and yellow 2.5 oz. bottle) add 1/2 to a full tank of gas & run. It took about 1/2 tank to completely clean the carbon from the cylinder. I added the other half of bottle and filled up again.
Backfire totally stopped. Now I use mid grade gas, no additive, and have no more backfire.
Might not be your problem however for $7.00 it is sure worth trying.
You need to call O'Rielly's to order RxP. Takes about 1 day to get it in.
Good Luck!
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