Carb icing (caused by the moisture in the fuel/air mix freezing as it passes the venturi in the carb) is a common problem with some bikes, but I don't think it is common with the XT600. I had it a lot on my Bandit (well-known issue with them) and it led to loss of power and stalling. No backfires. And it only occurred on damp and cold, clammy days. Strangely, it never happened in the rain.
The backfiring suggests unburnt fuel is being passed through the engine and igniting in the headers, which does suggest something is amiss with the spark - timing, or perhaps coil/HT problems.
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