Check you timing again. If it's backfiring then it's close. Remove the upper timing cover and rotate the crank til the timing dimple is facing the casting ridge hiding on the left side of the cam recess (if it's not lining up at the same time TDC is showing on the crank pulley, do a few rotations til it is aligned) Check your plugs while it's not firing and see if the plugs are looking wet with fuel which will indicate fuel delivery is working.