Classic/Antique Car Repair: Carb Backfires, dodge dart sport, air fuel mixture

Question
1975 Dodge Dart Sport with 318 V-8.  It backfires through the carburetor.  I've heard 3 possible scenarios for the cause and would like an expert's input.

1.  Mixture too lean (or was it rich?).
2.  Mistimed.
3.  Bad intake valve.

I have a guy locally that is willing to look at it but he's the one who mentioned the valve problem and estimated $1500 for repair.  If you could provide a short dissertation on why a car backfires through the carburetor, I'd greatly appreciate it!

Answer
There are two more common cause that you have not listed, a crack in the distributor cap. A rich mixture will not cause back fire up through the carburetor but lean will. A lean mixture causes a very high temperature in the engine and the cylinder will develop hot spots. These hot spots will ignite the incoming air fuel mixture when the intake valve is open and boom. On old cars this was also the cause of radiator overheating and can on older non catalytic equipped cars be spotted by a bone white ash in the tail pipe. The timing will usually not cause popping through the carburetor but a more common cause is the spark plug wires mixed up and out of sequence. The burned intake valve can be detected with a simple engine compression test. Low compression on one cylinder and you have your problem. The second one not on your list is a bent push rod or worn cam lobe on an exhaust valve. This prevents built up compression and exhaust from getting out of a cylinder and the next time the intake valve on that cylinder opens, pow.