I believe backfiring on decel is raw gas thats burning in the muffler.
I know on air injection systems, The "decel" vale diverts the air from the cat to atmosphere to keep extra oxygen from the mostly inert-gas exhaust stream(i.e. blowing up the cat) so I suppose an exhaust leak is possibly causing it. Diverter valves are known to fail often too.
On a different tokien, carburators leak and are notorious for drawing in gas on decel (high manifold pressure) so mabye the "air-bleed" circuit could be what to look at.(never mind-I forgot it was CIS.)
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