1989 Camaro Iroc-Z..backfiring

1989 Camaro Iroc-Z...We have replaced timing chain and gears, it is in time, but still backfires through the intake...What have I overlooked or what could it be? Thanks for any help.

firing order?

i had the same problem on my 86 i roc had to turn the dizzy 180 again and then it was fine