If it's getting fuel and it's getting spark, then air is the only thing left. If it's not getting air, perhaps something is wrong with the valve(s) for that cylinder? Have you popped off the valve cover to see if anything is visually wrong with the valves of that cylinder?

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'86 420SE Euro
904 Midnight Blue, Gray Velour
Dad bought it new, now I own it.

"A Mercedes-Benz is like a fine wine, it only gets better with age."