My 69 w115 has started to have a rough idle, sometimes missing at low rpm/idle. This has gotten worse over a 2 week period. I pulled each plug wire to see which was missing and sure enough, the most rearward cylinder had no spark at idle! Swapped plug wires and same thing. With the other cylinders I could see the spark jump and hear the lack of fire. When I idle up, the cylinder kicks in, stops missing and I see the spark jump. So what's the deal?

I replaced the coil with new points/condensor 8 months ago, along with new set of wires and plugs. Been running the car as a daily driver since.

What's the scoop? Where is my issue?
Why one cylinder over another? Worn distributor causing one out of 4 to be weaker?

Chris

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1969
Model 220 gasoline
Chassis: W115.010