If i had a wild guess, i would say it could be retarded. If the backfiring is really running on you may be too far advanced. I would try advancing first. To advance, turn the distributor clockwise. The revs will rise and the timing will advance. Once you have done this, turn off the motor and see if it still starts "easily". If it does, take for a drive and make sure it is not pinging(too far advanced) Still, try and remark the harmonic balancer and time correctly as someone had allready offered.
10.70 @ 125.00 on the black fun top
408 Cube Cleveland.