After changing spark plugs my car starts to misfire when I accelerate. What's causing that?
Basically when your engine misfires it means that one of the cylinders is not properly combusting fuel and air. This will cause the engine to feel shaky when you press the throttle down and will probably trigger a check engine light.
The most common culprit is the spark plug. The spark plug delivers the electrical current from the ignition system to the combustion chamber, and this is what ignites the compressed gasses. If the spark plug is old or has an incorrect gap between the electrodes it will not deliver the proper flame to combust the fuel properly. If the spark plug is not installed correctly or it is covered in oil, it will also prevent the proper flame spread needed for a good combustion event.
Other possible culprits are a bad fuel injector or malfunctioning oxygen sensor. These sensors are responsible for delivering the correct air to fuel ratio, but if they become clogged or malfunction then it can throw off the whole system. Also, if the engine is overdue for an oil change it could be another reason for the engine to start to misfire.
Lastly, it is also possible that something was disturbed or disconnected during the spark plug replacement. You may have reconnected the wrong plug wires causing one of them to fire at the incorrect time, or you may have not reset the spark plug gap when installing the new ones. It's always a good idea to remove and inspect the plugs visually (look for oil or soot on the electrodes) and to use a spark plug gap tool to ensure that the gaps are set properly.