How Do We Know It Is a Plug Wire? Let us assume a cylinder at idle showed abnormally high kilovolt, and the firing time seemed to be shorter compared to the other cylinders. We need to find out where that high kilovolt is created, internally or externally. Play with the throttle, accelerate and decelerate and watch the response. If the kilovolt stays high, this indicates that the greatest gap was external. How should the scope respond to rpm increase? At 2,000 rpm, the timing advance is between 30 and 40 degrees BTDC. At that time, the piston barely starts the compression stroke resulting in less compression when the plug fires. Therefore the reduced kilovolt tells us that the timing advance is functional and there was compression. We have verified the greatest gap is indeed in the combustion chamber and not external.
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