* Ignition Timing: This is the most likely culprit. Even a slightly advanced or retarded timing can cause backfiring, especially if it's inconsistent. Double-check your timing is set correctly according to the specifications for your engine. A slightly off setting might be fine at idle but create problems under load. Consider using a timing light to verify the timing at various RPMs.
* Distributor Issues: Problems within the distributor itself, such as a worn or faulty mechanical advance, a vacuum advance issue, or even a failing condenser, can lead to erratic ignition timing and popping.
* Carburetor Issues: While you have a new carburetor, incorrect adjustments (idle mixture, idle speed, etc.) can cause a lean condition, leading to backfiring. Make sure the carburetor is properly adjusted. A vacuum leak in the intake manifold or carburetor gasket could also be a problem.
* Vacuum Leaks: Leaks in the vacuum system can disrupt the proper functioning of the distributor's vacuum advance and also cause lean running conditions, leading to backfiring. Thoroughly inspect all vacuum lines and connections.
* Fuel Delivery: Although less likely with a new carburetor, ensure the fuel pump is functioning correctly and delivering the proper fuel pressure and volume. A clogged fuel filter could also restrict fuel flow.
* Camshaft Timing: While less probable given you have a new timing set, incorrect camshaft timing can lead to valve timing issues that manifest as backfiring. (Though this is more likely to be consistent, not intermittent).
* Exhaust System Restrictions: While less directly related to popping *through* the carburetor, a severely restricted exhaust system can cause back pressure that affects the engine's operation and contribute to intermittent popping.
Troubleshooting Steps:
1. Verify Ignition Timing: This is the first and most important step. Use a timing light to accurately set the timing.
2. Check for Vacuum Leaks: Use a vacuum gauge or spray carburetor cleaner around suspected leak points while the engine is running. A change in engine RPM indicates a leak.
3. Inspect the Distributor: Look for wear and tear, and ensure the mechanical and vacuum advance mechanisms are working correctly.
4. Adjust the Carburetor: If you're unfamiliar with carburetor adjustment, consult a repair manual or seek professional help.
5. Check Fuel System: Inspect the fuel filter and fuel pump for proper operation.
6. Exhaust System Check (less likely): Inspect your exhaust system for blockages or restrictions.
If you've checked these areas and the popping persists, it's advisable to seek the help of a mechanic experienced with older engines. A 396 is a powerful engine, and misdiagnosis can lead to further damage.
