* Timing: Incorrect timing can cause the engine to run rich, which can foul the spark plugs and cause a rough idle. Check the timing and adjust it if necessary.
* Fuel mixture: A rich fuel mixture can also cause black spark plugs and a rough idle. Check the fuel mixture and adjust it if necessary.
* Vacuum leaks: Vacuum leaks can cause the engine to run lean, which can also foul the spark plugs and cause a rough idle. Check for vacuum leaks and seal them if necessary.
* Faulty ignition system: A faulty ignition system can cause the spark plugs to not fire properly, which can lead to black spark plugs and a rough idle. Check the ignition system components, such as the spark plugs, wires, and distributor cap and rotor, and replace them if necessary.
If you have checked all of these things and the problem still persists, you may need to take the engine to a mechanic to have it diagnosed and repaired.
