My experience with the 1100GS is that if one doesn't have an oxygen sensor then it will backfire.
The reason is simple, you back off and the fuel is shut down. As the revs drop to a certain point the computer switches the fuel back on.
Because there is no oxygen sensor in the exhaust pipe, the computer cannot be told just how much or little fuel is going down the header pipe, therefore the computer drops a fair whack in to allow for error (so to speak) realises it's error and shuts it down again then opens the fuel again. This of course happens in milleseconds but the result is a bang or popping on moderate throttle openings.
It usually happens just as the tacho drops below 3,000 rpm.
Mick.