"2 things will effect the engagement; rpm and friction shoe spring strength."
Agreed .
"As I see it, how close the shoes are to the drum at rest is mostly irrelevant. "
So , you're saying that spring strength is irrelevant ? Weak springs will enable engagement at a lower RPM . If there were more distance between drum and shoes the engagement would happen at a higher RPM , all things being equal .
I have the original springs installed .
Warm idle speed seems fine , at the moment . It's when cold and still on partial choke that the idle is a bit higher and the transmission wants to engage . I suppose it's possible that after I get the carb fine tuned that it might 'settle down' a bit .
And , I'm sure that after the shoes wear a bit , it may not be as much of an issue .