Wasted Spark Long Tail mystery

Hi there, anyone have an insight/suggestion into the cause of the very long tail on the Wasted Spark sec ig line for 0004 cylinder? No misfires evident and idles and runs well. This has new plugs and wires, but the long tail was evident before the parts refresh. The car is a 1990 NA Miata 1.6lt fully stock. Interested in views. Long tail on not evident in other cylinder Wasted Sparks. Cheers, Alex

Is this what you are referring to?



Waste Spark coils fire two spark plugs, simultaneously. One fires with positive polarity, and the other with negative polarity.

The negative polarity firing event will be inverted. I suspect that this is feedback of one of those events, from another coil.

Also, you might consider increasing your your resolution. You will get a better picture.

Hey Chad, thank you for the reply and the insight. The waveform is the same cylinder firing on comp and exhaust, hence the firing lines - in this instance - are both positive. If I understand you correctly, you believe the long negative tail may be interference from the companion cyl 1, which is on firing on compression at the time that cyl 4 is on exhaust? If I've got you right, then I think, yes, that's possible. The #4 wire is extremely short and runs right next to the #1. I'm wondering what I can do to confirm. I may play around with the inductive pick and see if it eliminates the long tail, or run a temporary longer wire and route it clear of the other wires. I left the resolution low because bumping it up to 12 clips the the peaks of the firing lines and I wanted you to see the extent of the long tail. Cheers, Alex

If you look very closely at the trace you also see the sparks for the other two cylinders one of which is also inverted ---- this is perfectly normal.