Oh dear .. again.. BIG problem

OK, not in a happy place right now, but for the purposes of documenting, again I hope I will look back at this and smile. The sequence of events is tied to previous entries. Annoyingly I had Amy running quite nicely again. Smooth pull away, fairly smooth running. The issue was that after the cleaning out - the Carb was running richer than before. Adjusting the mixture had the desired effect. Very happy at that point. I adjusted a bit leaner for another test run, and it felt equally OK, so did a bit more of a run - up to 50 mph just to check.

Got a couple of violent backfires, so figured I'd gone too lean, so adjusted back again.

New problem, now running really lumpy. Assumed it was still fiddling with the mixture that was required, but after several tests, no difference.

Also this morning more fiddling, no change.  Felt like it was running on 3 1/2 cylinders. Did a cursory check on compression by turning over with the starting handle. One pot was a LOT easier than the others. Sure enough, when taking out the spark plugs, the one in pot 4 was black and oily. The picture doesn't really show it too well, as I wiped it before photographing it - but that's the one at fault.

Now, that pot was always a bit blacker than the others, it seemed to be slightly oily and there was always a bit of oil in the water, I had a sneaking suspicion that it needed doing anyway, was just in denial. So I hope that I have only accelerated the need to do it and it's not something else.