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Check alternator and make sure you have enough spark to fire the engine with the plugs in the cylinders/under compression. Perhaps not enough voltage (could be bad or weak grounding too).
I have had backfiring in the past related to low voltage/not enough amps to fire the plugs under load...
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Emery
1988 930 coupe - Silver Metallic
TurboKraft 3.3L 8:1 CR, SuperSC Cams, GT35R, B&B Headers, TK intercooler, Tial WG, ARP, tecGT based phased sequential EFI & ignition, Wevo shifter/coupler, ... 
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