As far as I can tell, it’s correct. Bottom line is, with spark advance at slow or retard, the carb butterfly is closed no matter what twist grip setting. With spark advance at WOT, the carb butterfly is free to be moved by the twist grip.
While cruising along at WOT and twist grip on Fast, it should sing along. Leave the spark advance at WOT and twist he grp to Slow and the carb will close, causing it to slow down, albeit rough running. Twist to Fast and away you go again.
But for smooth idle/slow running, the carb must be closed and spark retarded, all done automatically by sliding the s/a lever to Slow.
PS, think race motors with dead-man’s throttle
