Eureka!
It was screaming pickup to me, but it doesn't help when people tell you that pickups either work or don't work.
So what we have learned here is that not only can a pickup failure manifest when it gets above a certain temperature, but also when engine revolutions exceed a very low threshold.
This proves that they
can work just a little bit, but not enough!
I'm delighted you've cracked it and without mauling the carbs too.
You only ever find what you're looking for in the last place you look unless you insist on continuing to look after you've found it..
