Have a 600 cfm Holley 4160 on a 77 F-100 step-side with a 400. I searched and posted here (and other forums) for a fix to my carb backfire problem. Got lots of good ideas and tried several: adjusted the float level and air mixture screws, looked for vacuum leaks, adjusted the vacuum advance, rebuilt the carb, and rejetted - all with no success. Was almost to the point of dumping the Holley for an Edelbrock due to Holley's tendancy to have backfire problems (try searching this forum on "Holley" and "backfire") But, it turned out to be TIMING. I had tried different timing settings from 0 BTDC to 30 BTDC, when I tried ~1 deg. increments. Turns out it backfires at 12 BTDC, but runs like a banchee at 15 deg. BTDC. Finally got my truck back. Thanks for all the help!