A friend of mine with a 347 Ford has a backfiring problem. When he gets on it and then lets off at the top of a gear, it backfires once through the exhaust. We datalogged the A/F, and it has an off-scale lean spike right at the time of the backfire. We just got it running a few weeks ago, and we put Proform metering blocks and a Proform main body in the carb (not my idea). It doesn't do it with a brand new 750 Demon on it, so it must be the carb or the tune in it. I have my own theory as to the cause of the problem, but I'd like to hear your ideas first.