Also try re-setting the mixture screw on the under side of the carb. I'm not familiar with XT's but you'll probably find it on the the cyl.head side of the carb. If so, it will probably be controlling fuel at tickover. Screw it in finger tight & no more and then screw it out approx. 1 to 1.5 turns. Try this on a hot engine. the backfiring should decrease. If not screw it out in eighth of a turn increments. If it still does not cure it by the time you're 3 or 4 full turns out, then it is a leaking exhaust or carb manifold.
If trying this, it is worthwhile taking the screw right out, watch out for the spring (sprung loaded on some bikes) & greasing it prior to screwing it back in finger tight (no more!!). Seized mixture screws are a pian in the arse.
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