Exhaust backfire

[66DARTGTMAN]

I just finished installing a new Summitt carb on my 66 dart with a 383 in it. I have to crank it a lot and it will fire when i let off the starter. Then it wont idle I have to keep the rpms around 1500 to keep it running. Anything under 1500 and it wants to die. Could this be timing or the air/fuel needles on the new carb. What would be a good start for the timing degrees. Am I correct in thinking that the backfire is unburned fuel in the exhaust.
Thanks in advance

 

[67Dart273]

Sounds to me like you have a big vacuum leak. Or maybe the idle screws need to be richer (turn out) or the idle circuit has a problem

Timing? Is this a new build? The timing should not have changed from the carb you took off. Otherwise the timing depends on the cam you have in it.