When dose it do it when you floor it or when you drive it up the revs slow . I ask this as my 3dr had this for a long time it would drive spot on all the way up the revs but when i booted it it would back fire hi up in the revs . It turnd out to be the leads were bracking down and they were to close togever when i moved them apart it sorted it i did put a new set on when i found this out . im all so running wasted spark