Ok its been awhile since I have been on, but I have a 79' Z with a fresh 350. Anyways it is backfiring through the exaust at idle... Its not that bad but pretty consistant. Do I need to retard or advance the timing? Or could it be that its cold outside and I have no exhaust behind the full-lenght headers? I have checked my plugs, exhaust manifolds at the heads and internal drivetrain.

 

Could it just be the idle mix on your carburetor? If you're too lean, that exhaust charge'll ignite. What's you timing at now?

 

Double check your plug wires. I'm am extremely careful, patient guy, but I once had a couple plug wires switched around, and my engine backfired consistently.
Also, position the engine at TDC for the #1 cylinder and check the relationship of the cap and rotor. If you have a Holley or Edelbrock carb, make sure the idle mixture screws are out approximately 1.25 turns. I guess if all that doesn't help, check the fuel pressure and float level.