Backfiring at high rpm is something I would associate with a lean condition. Just because your seemingly rich at idle or low speed conditions doesn't mean your not lean at the higher rpms under load. Power valve in the carb would be suspect, IF you have one, not all carbs do. As for the burning eyes, it's a bit of a myth that is always a sign of a rich condition, lean burn can cause the same symptoms. NOX is what you smell and what makes your eyes burn, a LEAN condition generates a LOT of NOX.
As to the black tail pipe, I guess I could fly out there and stick my finger up your tail pipe to verify, on second thought, I'll take your word for it.
