backfiring or misfire

[TRANTER]

when i accelerate the car starts to pull fine till around 4,000 revs then pops and backfires, also when sat still and reving it backfires realy loud sometimes any ideas what this could be

done so far : oil
                  filter
                  leads n plugs
                  coil
                  battery
                  dizzy
                 
any suggestions would be fine i havent took it out on a good drive for months if that helps

[Uruk Hai]

Ignition timing ?

[TRANTER]

could they just jump out of timing as it hasent realy been anywhere and a few weeks ago it was fine