It's not always lean on top end that causes a backfire. Mine seem to be more prone to backfire comming up off idle.

You can have a lean idle and it will idle all day but if you jam the throttle up, it will pop and it's all over.

Play with your engine some and enrichen the idle a little. You may have to run your trim up some to keep it idling?

Also bring your throttle up slowly to keep back fires down.

Good luck,

Jim