* Rich Fuel Mixture: This is the most likely culprit. The engine is getting too much fuel relative to the air. Several things can cause this:
* Faulty Fuel Injectors: One or more injectors might be stuck open or delivering too much fuel.
* Clogged Air Filter: A restricted air filter reduces the amount of air entering the engine, leading to a rich mixture.
* Vacuum Leaks: Leaks in the vacuum system can disrupt the air/fuel ratio.
* Mass Airflow Sensor (MAF) Problem: A malfunctioning MAF sensor provides inaccurate air readings to the computer, resulting in incorrect fuel delivery.
* Throttle Position Sensor (TPS) Problem: A bad TPS gives the computer inaccurate information about throttle position, leading to improper fuel delivery.
* Faulty Computer/ECM: The engine control module (ECM) itself could be malfunctioning, leading to improper fuel delivery.
* Incorrect carburetor settings (if applicable): If your 400 has a carburetor, incorrect jetting or other adjustments could be causing a rich mixture.
* Low Compression: While less likely to cause *only* black smoke, low compression in one or more cylinders can lead to incomplete combustion, resulting in unburned fuel being expelled as black smoke. This often presents with other symptoms like misfires and low power.
* Timing Issues: Significant timing problems, particularly if the timing is retarded too far, can lead to incomplete combustion and black smoke. However, this is usually accompanied by other performance problems.
Troubleshooting Steps:
1. Check the air filter: Replace it if it's dirty.
2. Inspect for vacuum leaks: Look for cracks or loose connections in vacuum lines.
3. Check the fuel pressure: This requires a fuel pressure gauge. Too high or low fuel pressure can indicate problems.
4. Have the fuel injectors tested: A professional mechanic can usually test the injectors for proper spray pattern and flow.
5. Inspect the MAF sensor and TPS: Have them tested or replaced if necessary.
6. Check the engine's compression: A compression test will reveal if there are any problems with the cylinders.
7. Inspect the spark plugs: Sooty or wet plugs often indicate a rich mixture or incomplete combustion.
Without more information about the engine's condition and other symptoms, it's difficult to pinpoint the exact cause. It's best to take your Chevy 400 to a qualified mechanic for proper diagnosis and repair. Continuing to drive with a rich fuel mixture can damage your catalytic converter and other engine components.
