The correct arc for the Plymouth spark plug is not something that just happens because you want it to. The entire ignition system must be operating as it was designed to, along with the spark plug being the correct type with the right gap setting to make this component ignite the air and fuel mixture completely. It is the combustion of the air to fuel mixture aided by the spark plug that propels the engine's internal components to rotate. There is one for each cylinder in your Plymouth engine. Each one must be able to produce a similar level of ignition for the overall operations of your engine to be as productive as possible. Circumstances that can interfere with the proper operations of your Plymouth spark plug include oil leaking into the cylinder fouling out the plug. The electrical impulse through the ignition wire can also be degraded to the point that a good spark cannot be initiated by the spark plug. The most common reason for a spark plug in your Plymouth engine not to operate properly is that it is just worn out. This eventually happens to all of them. It is the electrode at the end of the plug that will deteriorate in time from being used. With each jump of the electricity in creating the arc, the end of the electrode will lose some of its material. As this occurs, the gap setting of the plug will increase; in time, weakening the effectiveness of the arc and its ability to ignite the air and fuel mixture in the combustion chamber. In time your Plymouth engine will not operate correctly and could even fail to start if a regularly scheduled tune up is not routinely conducted.