Ignition System:
* Worn Distributor Cap and Rotor: These are common wear items. Cracks or corrosion can cause misfiring and backfires, especially under deceleration when vacuum changes are significant.
* Worn or faulty Ignition Coil: A weak coil might not provide sufficient spark under the lower voltage conditions of deceleration.
* Worn or faulty Spark Plug Wires: Cracks or poor insulation in the wires can lead to arcing and misfires, especially at higher RPMs then the spark jumps across a weak point on deceleration.
* Point gap (if equipped): If your Nova still has points, an incorrect gap or worn points will cause erratic ignition timing.
* Vacuum Advance: The vacuum advance mechanism in the distributor can malfunction, causing incorrect timing during deceleration. A leak in the vacuum line can cause similar problems.
Fuel System:
* Carburetor Issues: This is a very likely culprit. A carburetor problem is most likely to manifest as a backfire on deceleration. Possible problems include:
* Lean Fuel Mixture: Too little fuel during deceleration can cause the unburnt fuel to ignite in the exhaust.
* Vacuum Leaks in the carburetor: Leaks can affect fuel/air mixture and timing.
* Sticking Choke: A choke that doesn't fully open can cause a rich mixture, but it's less likely to cause backfiring *on deceleration*.
* EGR Valve (Exhaust Gas Recirculation): If the EGR valve is stuck open, it can cause a lean mixture and backfiring.
* Fuel Pressure Regulator: While less common, a malfunctioning fuel pressure regulator can lead to inconsistent fuel delivery.
Other Possibilities:
* Exhaust Leak: While less likely to *cause* the backfire, a significant exhaust leak *before* the catalytic converter (if equipped) could allow unburnt fuel to escape and ignite, potentially contributing to the sound.
* Timing Issues: Incorrect ignition timing can cause backfires under certain conditions.
Troubleshooting Steps:
1. Start with the easy stuff: Inspect the distributor cap, rotor, and spark plug wires for cracks, corrosion, or wear. Replace them if necessary. Check the spark plug gap.
2. Check the carburetor: A thorough cleaning and adjustment may be necessary. Pay close attention to the idle mixture screws and vacuum connections. A mechanic specializing in carburetors would be helpful here.
3. Inspect vacuum lines: Look for cracks or leaks in all vacuum lines connected to the carburetor, distributor, and EGR valve.
4. Check the ignition coil: Test the coil's output with a multimeter if possible, or have a mechanic test it.
5. Test the EGR valve (if equipped): Check for proper operation and vacuum signal.
Important Note: Backfiring can be dangerous. If you're uncomfortable working on your car's ignition or fuel system, it's best to take it to a qualified mechanic. Ignoring the problem could lead to more serious damage.
