Back in the sixties before rice-burners became the transport of choice over here in the UK and when most self-respecting bikes were Brit singles, this was a perennial problem. The received wisdom then, and I suggest it hasn't changed, is that overrun backfires are almost always caused by exhaust leaks - sometimes impossibly tiny ones.
The theory is that some unburnt fuel vapour invariably escapes from the combustion chamber on overrun, but, because of the absence of oxygen in the exhaust pipe, it can't ignite. Introduce a leak (along with enough oxygen) and you get the mini explosion inside the exhaust we call 'backfiring'.
Now, you can debate this all day and get all bent out of shape over it, but every time I've had a big single start to backfire, I've ALWAYS gone to the exhaust plumbing, carefully remaking each joint in turn, and had the problem resolved as a result.
The Skorp's no exception to this useful rule of thumb and, on the 2 occasions I've had backfiring occur, I've traced the problem to the joint between the downpipe and silencer can. I've always fixed it using aluminium foil and silicone bathroom sealant, which hardens nicely to form an neat and airtight seal.
I have seen occasions where carburation has caused backfiring too - but that's always been where someone with the mechanical sympathies of an alcoholic chimp has had a thorough futzing session with the jetting. I suspect that's not the case here (but then I'm just in a good mood 'cos the sun's shining again.)
