* Ignition Timing: This is the most probable cause. At higher RPMs, the ignition timing needs to be advanced to ensure the spark occurs at the optimal point in the combustion cycle. If the timing is too retarded (too late), the unburned fuel ignites in the exhaust system, causing the backfire through the carburetor. This is especially common with a worn distributor or a faulty vacuum advance mechanism. The vacuum advance helps adjust timing based on engine load.
* Distributor Issues: Several things can go wrong with the distributor:
* Worn points (if equipped): Points wear down, causing erratic or weak sparks. '79 models might still use points, or they might have an electronic ignition.
* Worn or failing condenser: The condenser helps the points break cleanly and prevent arcing; failure causes weak or missed sparks.
* Worn rotor: The rotor distributes the high voltage to the spark plug wires. A worn rotor can cause poor contact and misfiring.
* Worn or damaged cap: Similar to the rotor, a cracked or corroded cap will lead to poor spark delivery.
* Failing ignition module (if electronic ignition): The module controls the spark timing and strength. A failing module often causes problems at higher RPMs.
* Worn Spark Plugs or Wires: Worn spark plugs can't effectively ignite the fuel-air mixture, leading to incomplete combustion and backfiring. Similarly, cracked or corroded spark plug wires will lose voltage, resulting in weak or missing sparks.
* Valve Timing: Although less likely to be RPM-dependent like the ignition issues, incorrect valve timing (due to a slipped timing chain or incorrect adjustment) can cause backfiring. This would usually cause backfires both above and below 3000 RPM, but it's still possible the issue manifests more severely at higher speeds.
* Carburetor Issues (Less Likely): While a carburetor problem *could* theoretically contribute, it's less likely to be the primary cause of backfiring *only* above 3000 RPM. A lean fuel mixture might contribute, but the ignition issues listed above are far more probable.
Troubleshooting Steps:
1. Check the ignition timing: This is the most crucial step. You'll need a timing light to verify the timing is correct at various RPMs, especially around 3000 and above.
2. Inspect the distributor: Carefully examine the distributor cap, rotor, and points (if applicable) for wear or damage. Replace any worn components.
3. Check the ignition coil: Ensure it's providing sufficient voltage.
4. Inspect spark plugs and wires: Replace any that are worn, cracked, or corroded.
5. Check the vacuum advance: Make sure the vacuum advance mechanism on the distributor is working correctly. A disconnected or leaking vacuum line can cause significant timing issues.
6. (Less likely) Examine the valve timing: Verify that the timing chain or belt is correctly aligned and in good condition. This requires more advanced mechanical knowledge.
Start with the simplest and most likely causes (ignition timing and distributor components) and move to the more complex ones. If you're not comfortable working on your engine's ignition system, it's best to take it to a qualified mechanic. Improperly working on the ignition system can be dangerous.
