Not a carb expert by any means, just a thought....
Usually backfire is excess fuel.
What jets are you running?
Here's a good read from TY.
TY Forums - jetting....why needles are overlooked
A Motto to Live By:
Life is not a journey to the grave with the intention of arriving safely in a pretty, well preserved body,BUT rather to skid in broadside, thoroughly used up, totally worn-out and loudly proclaiming
WOW , WHAT A RIDE !