Close ! your too lean , not very often would you look to the valves as a cause as Suptter said.
The only time I have gotten a backfire on any of my Saito's was they were too lean MY FAULT.
Another thing is to look at amybe the prop size 18/6W is about the top of the prop list .
I have had a backfire with a prop on the big side of the engine range on spool down (CHOP Throttle) BANG! a heavy prop and leanish condition will do that too.

Use a Tach



Den