I had a VS Commodore -T4 Lexcen 3800 with Impco mixer set up which kept backfiring if you weren't ginger with the throttle. Drove me batty and my local installer rerouted a couple of hoses which stopped it for a while before it eventually reoccurred.
Then a guy on another forum I frequented told me of a cabbie with a VX Commie who gapped his plugs to 0.8mm which he claimed stopped this well known issue on his car. When I did it to mine it worked like a charm and also improved throttle response no end. It had a retarded feeling from a standing start before I regapped the plugs. Made for an enjoyable driving experience with decent fuel economy of 12-13L of gas per 100km in mixed freeway and urban conditions.

Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!