D2's will always backfire on LPG unless you set the plug gaps to 0.6mm/0.024". The coil packs are very prone to crossfiring unless you do this. Also make sure the leads aren't tied into a bundle, use spacers or split corrugated tube to keep them separated.
And if it still backfires you may need replacement coils, I suggest original second hand ones from a petrol-only wreck.


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