New motor backfiring through carb

Try this: Remove #1 spark plug. Put a threaded compression gauge or Sharpie marker or a friend's thumb in the spark plug hole. This is the compression stroke. Keep rotating the harmonic balancer bolt clockwise until the balancer line is on '0' [TDC]. Remove the distributor. Look down the distributor hole. The slot in the gear shaft should point to the left front intake manifold bolt. If not, pull up on the gear and turn it till it does. Note that the gear teeth will rotate the slot as it is fully seated. Also, a long straight screwdriver or tool may be necessary to turn [reposition slightly] the hex in the oil pump, as the gear and the hex must align for the oil pump shaft to seat fully. Drop in the distributor with the rotor tip pointing to the front of the car, and the vacuum advance unit nipple pointing to #8 spark plug [or 12 and 3 o'clock from the driver's seat]. Snug down the dist. clamp so you can turn the dist. by hand. Recheck plug wire location. Fire it up. Set timing. Road test!