backfire

[cornhilldo]

My 1100 GS occasionally backfires, usually after a period of sustained speed, when I throttle off. There is also some soot deposits at the exhaust outlet below the opening. And when in town, when hot, it won't hold a constant throttle at lowish revs. Could it be running a bit lean, if so how can this be sorted. I've been recommended a power commander, but they are a hell of a price for bm's.
my bike is standard, '97, 17,000 miles, doesn't seem to be using any oil, getting 41-42 mpg and showing 5 bars when hot.
Any thoughts,
thanks

 

[Big Den]

Could be a leak on the exhaust, mine does it to order and thats whats causing mine.