Alright, finally got time to muck around with the mini this weekend,

Turns out if the lead was slightly off cylinder 4's plug, it was firing, however if the lead was completely on the plug, it wasn't... Weird.

Went and bought some new NGK BP6ES's and problem solved!

I had initially swapped plugs, leads, etc around before that, and the issue didn't follow any of them, so it was definitely weird to find the fault was a spark plug.

I now run BPR6ES. The resistor seems to help stop fouling by making the coil work harder (maybe, IANAE).
Anyway, I've not fouled a plug since I changed to these 3 years ago at GR's suggestion. Running with the 45 Dellorto, the plugs stay clean, in traffic or not.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Yeah Doc,

It will be interesting to see the colour of the plugs after a few weeks to see if there are any other issues. Fingers crossed it's all good. The previous owner was running BP5ES's, not sure why... They were hotter, so I have a feeling they could have been contributing to the engine running on a few times.

Took the carbies off again, the linkages were bent and preventing the accelerator lever from returning to idle properly. Just waiting on the new linkages, at least this way i can get rid of the horrible welding the previous owner did to lock the linkages in place...

Once they're back on, hopefully with mixtures all good, the spark plugs will stay clean!

Heat range 5 are recommended these days for use with unleaded fuel, particularly in daily driving. Less likely to foul.
I run 6s though because of the 11.0:1 C/R.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.