Sunday ride went bad, At higher rpm's engine misses, backfires. Idles and runs good at lower rpm's. Thought it was a fuel issue, but after removing and inspecting the S&S super B, found no dirt or debris in the fuel system. The ignition was changed to a magneto when built last june. Had been working great till now. Has anyone ran across these symptons before? Not sure if timing advance or fuel problem was over looked???? HELP