Is yer frottle spring in the right place ?
Yes thanks for asking.
I have checked that the throttle goes fully wide when fully actuated, and closes properly to the tickover rest position when released.
Since I last posted, I have, this morning, taken the carb totally apart and blown through all orifices with brake cleaner, carb cleaner not being available. Not much muck if any came out and all gaskets, diaphragm etc seem to be in good nick.
also burnt off the carbon on the spark arrestor, as far as I could.
Put it all back together and it is behaving exactly as before, i.e. apprently flooding as it will only run when I deflood it and start it, then it runs like mad for a few seconds before stopping.
I am now thinking it must be the plug, even though that seems to pass all continuity tests and doesn't seem to have a short due to cracked insulation. The gap is correct, it was only a fraction too wide.
(Received wisdom is to change them every year and it is still the original.)
It is behaving as if it is flooded, that seems to tell me that it isn't getting a spark sufficiently big to start it with a richer mixture. The mixture settings not having ever been played with, (although I admit that having made and fitted an air filter that will have richened the mixture a bit, so when testing I left it off) indeed I couldn't even alter the needle screws until I did the butchery yesterday.
The only thing I will not be able to play with will be the timing.