Fuel is chopped completely on the over-run down to, (I believe.) 2,700rpm so at 4.6-4.8 TPS reading this means that when the throttle is closed no fuel is delivered until the engine slows to 2,700 RPM. No fuel? No backfiring. With the factory maps idle fuel is delivered all the way down, no matter what the TPS reading. At certain points the pipe harmonics will make the engine 8 and 16 stroke so the mix burns in the pipe. Hence the backfiring.
Pete
Just noticed this answer to my question about fuel shutoff on decal. Thanks.
