Yo Ray,
Nine times outta ten it's an exhaust leak. Extra oxygen gets into the exhaust flow somewhere after the exhaust port and mixes with the unburned fuel. Happens on deceleration. BOOM!
Figure out where it is leaking and tighten or replace the gasket as required. I always try tightening first (don't overdo and break something off).
Seems to be a common Cobra disease caused by vibration and heat
Tom