Is it backfiring through the intake or exhaust? If it's a induction backfire, it could possibly be a timing and/or lean issue. If you have an exhaust backfire, your engine could be running really rich. Is your check engine light on? Have you checked your base ignition timing? If you go to corral.net, they have an article about retrieving diagnostic trouble codes, using the check engine light by just jumpering the diagnostic connector with a wire, paper clip or whatever you have handy.
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2005 Suzuki Hayabusa GSX1300-R
1980 Ford Thunderbird - 255 V8
ported heads, 5.0L ported stock headers, O.R. H-pipe and Flowmaster 2-chambers, dual roller timing chain
hi-po Mack Truck hood emblem 
1985 Mustang GT 5.0L T5, F-303, GT40p, headers, off-road h, flowmasters, MSD stuff, etc.
Sold 02/06/04 
1989 Mustang GT ET: 13.304@102.29 mph (5-24-03)
Sold - 1998 Mustang Cobra coupe, 1/4 mile - street tires: 13.843@103.41 (bone stock)
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