Past experiance
did you make sure they was the same reach and everything,because other than them not being gaped right it should have run pretty good,go back over the firing order those are easy to cross up,one plug wire crossed will cause this,and you may have did like i did once, i bought a bad set of spark plugs,people say it cant happen,but it can,i bought a new set of ac delco plugs for a v-6 engine,and 2 of them was dead as a door nail and it made my truck back fire real bad,id go back over it and check it good,anythings possible,good luck with it.
did you make sure they was the same reach and everything,because other than them not being gaped right it should have run pretty good,go back over the firing order those are easy to cross up,one plug wire crossed will cause this,and you may have did like i did once, i bought a bad set of spark plugs,people say it cant happen,but it can,i bought a new set of ac delco plugs for a v-6 engine,and 2 of them was dead as a door nail and it made my truck back fire real bad,id go back over it and check it good,anythings possible,good luck with it.
