Hey,
I've got an '81 XT550 and it's backfiring alot, mainly when I come off the gas. It also does it when I kick it over whilst holding the decompression lever, usually just after an unsucessful attempt to start - it always starts on the second or third kick both hot and cold. The bike has always backfired, I'm sure most of the older big singles always have but these far more aggressive sounding, super loud buggers!
Any kind of help is appreciated, cheers.