backfires under full throttle while shifting into 4th

[HarleyFordFan]

my car just started having a little dilema.it backfires under full throttle while shifting into 4th gear (the only time it does this). what is causing this? my mods are listed in my signature. BTW timing is set at 12* if that matters much. could my timing be off? what should i set it at? how do i do it? also would a fresh tune-up cure this?