@Stanleysteamer well, well, well. Here is our SU carb "expert tuner" showing us all a perfect plug after a tuning session.
Just like the guy in the YT clip advised look at the fire mark/band on the earth strap of the plug. 100thou' band just above the thread of the plug.
View attachment 347138
On what is likely to be a final remark to do with this as I think you have got it aabout right now, this trubble you have had reminds me very vividly of the same trouble, sort of, that I had with W's Porsche copy kit car.
The engine was a tuned 1800 version of a VW flat four aircooled. Built by a guy called Arnie Levics, who apparently is a VW guru, and this was done for the kit car company when they sold ready built vehicles.
It has twin Webers with vertical intakes, a sports exhaust, high lift rockers, etc and it had a Bosch 009 dizzy, (no vacuum advance). It ran fine EXCEPT when warm and you wanted to take off from a standing start, like at lights, a roundabout or turning right across traffic, when it turned into a death trap. Rather than pick up when the accelerator was depressed, it would either bog down or stall.
I did a lot of research and learned that this was common with these carbs.
So I thought I'd obtain a normal dizzy that uses intake vacuum to advance the spark.
Not a problem, I got a new dizzy and even made it electronic. But I couldn't get it to respond to vacuum, either from the vacuum take off, off one of the carbs, or even from inlet manifold vacuum.
So in quiet desperation, I built myself a distributor testing machine. Very Heath-Robinson, but it worked. And with that I worked on modifying the total amount of advance, and how fast it came in, i.e. the advance curve.
I learned a lot by doing this, one of the things being how easy it is to measure advance by removing the springs and moving the central part of the dizzy that the rotor arm attaches to, if you make scratch marks or similar on the top edge of the body of the dizzy, lined up with similar on the rotor arm.
I eventually got it running better but I was still never happy with it.
But much of what you have been going through reminded me of my tribulations with it!
Personally I do not like the aircooled VW engines. I know they have their addicts. But I would be happy if I never had to work on one again.
Tinkering with BL A and B series engines is so much more gratifying!!