Ignition System Issues:
* Timing too far advanced: If the ignition timing is too far advanced, the fuel-air mixture ignites too early, causing a backfire through the exhaust. This is especially noticeable under load.
* Worn distributor cap and rotor: These components distribute the high voltage to the spark plugs. Wear or damage can cause misfires and backfires.
* Worn or faulty spark plugs or wires: Poor spark delivery leads to incomplete combustion and backfires. Check for cracks, corrosion, or excessive wear.
* Ignition coil problems: A weak or failing coil won't provide sufficient voltage, resulting in weak sparks and misfires.
Fuel System Issues:
* Lean fuel mixture: Not enough fuel relative to air. This can be caused by a clogged fuel filter, faulty fuel pump, problems with the carburetor (common on a '73 350), or incorrect carburetor adjustment.
* Vacuum leaks: Leaks in the vacuum system disrupt the air/fuel mixture, leading to inconsistent combustion and backfires. Check all vacuum hoses and connections.
* Clogged catalytic converter (if equipped): A severely clogged catalytic converter can restrict exhaust flow, causing pressure buildup and backfires. This is less likely in a '73, but possible depending on modifications.
Exhaust System Issues:
* Restricted exhaust: A severely clogged muffler, tailpipe, or other component can cause back pressure that leads to backfires.
Other Potential Causes:
* Valve timing issues: Incorrectly adjusted valve timing can affect combustion and cause backfires. This is less likely unless there's been recent engine work.
* Intake manifold leaks: Similar to vacuum leaks, intake leaks disrupt the air/fuel mixture.
Troubleshooting Steps:
1. Start with the easiest checks: Inspect the spark plugs, wires, distributor cap, and rotor for wear or damage. Replace any faulty components.
2. Check the ignition timing: This usually requires a timing light. If you're not comfortable doing this, take it to a mechanic.
3. Inspect the vacuum lines: Look for cracks, loose connections, or leaks. Replace any damaged lines.
4. Check the fuel filter and fuel pump: Ensure they're functioning correctly.
5. Listen for exhaust leaks: A loud exhaust leak near the manifold or headers can contribute to backfires.
Important Note: Backfires can be dangerous. If you're not comfortable working on your car's engine, take it to a qualified mechanic for diagnosis and repair. Improper diagnosis and repair can lead to further damage or injury.
