I have an 86 Heritage that has started to backfire at about 3000 RPM. I have an S & S Super E carb and a Crane Hi-4 single fire ignition. It's backfiring through the exhaust with a flame coming out of the pipes. It runs fine in lower RPMs but as soon as I get to 3000 RPM in any gear it will continually backfire. The only time it doesn't backfire is when in neutral and I rev it up. Any ideas?