* Incorrect Firing Order: The most probable cause. If the spark plug wires are connected to the wrong cylinders, the engine will misfire badly, potentially leading to a backfire through the carburetor. Double and triple-check your firing order against a reliable source for your specific engine (a shop manual or online resource for your year/model is crucial). Even being one cylinder off can cause this.
* Wires Crossed or Damaged: Examine each spark plug wire carefully. Make sure none are:
* Touching each other: This can cause cross-firing and misfires.
* Damaged or cracked: A damaged wire can cause a short circuit or inconsistent spark delivery.
* Loose on either the spark plug or distributor cap: Ensure a tight and secure connection at both ends.
* Distributor Cap/Rotor Issues: While less likely if the wires are newly replaced, inspect the distributor cap and rotor for cracks, corrosion, or worn contact points. A faulty distributor cap or rotor can disrupt the spark delivery sequence.
* Vacuum Leaks: A significant vacuum leak can disrupt the air/fuel mixture, making the engine more prone to backfires. Check all vacuum lines connected to the carburetor and intake manifold for cracks, loose connections, or damage.
* Timing Chain/Belt: A significantly off timing chain or belt (though less likely to only cause this specific symptom) could also lead to misfiring and backfires. However, if it *were* the timing chain/belt, you'd likely have a lot more than just backfiring through the carb.
Troubleshooting Steps:
1. Verify Firing Order: This is the absolute first step. Get the correct firing order, then meticulously check your wire connections. Use a diagram and carefully trace each wire from the distributor cap to its corresponding spark plug.
2. Inspect Spark Plug Wires: Look for any damage, loose connections, or wires touching each other. Replace any questionable wires.
3. Check Distributor Cap and Rotor: Inspect for cracks, corrosion, or wear. Replace if necessary.
4. Inspect Vacuum Lines: Look for cracks, loose connections, or damage in all vacuum lines.
5. Re-check Everything: After addressing the above, double-check all your work before attempting to start the engine again.
Safety Precautions:
* Do not crank the engine excessively if it backfires repeatedly. This could damage the engine.
* Work in a well-ventilated area. Carbon monoxide is a serious danger.
* Disconnect the negative battery terminal before working on the ignition system.
If you've checked all of the above and still have the problem, it's best to consult a qualified mechanic. There could be a more complex issue, such as a problem with the carburetor itself or the ignition control module.
