Not Ranked
Turk,
Most of the time you get backfires when you deaccelerate is due to air leaks in the headers. Tighten all the header bolts when the engine is cool and see if that helps. If it doesn't consider replacing the gaskets and and retightening the headers. IF your timing is correct, most of the time these air leaks are the cause. When you take your foot off the gas, the increased vacumn draws air in through the leaks and the oxygen in the hot headers then fires the raw gas that was not burned in the cylinder. This problem is usually worse if you are running an overly rich mixture, but based upon your plug readings this appears not to be the case.
bkozlow
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