[Jeff Chambers]

Exhaust backfire = exhaust leak. There will always be some amount of unburnt fuel in the exhaust stream. The backfire is caused when you introduce an amount of air sufficient to create secondary combustion. Look for exhaust leaks, particularly downstream of the collectors. This is the point where the dynamic pressure is lower than ambient pressure, causing the exhaust to 'suck in' fresh air. This is the same reason that an exhaust leak will cause an A/F meter in the tailpipe to read lean.