Interesting.
I use soft starter cone that doesn't slip.
Also, how rich are you running?

Try this.
I always start my YS engines by turning the prop clockwise so that the prop has some space before getting to compression then attach glow driver, then use Kavan starter. It is a lot easier to turn that way. (using momentum?)

Are your starter slip on the spinner cone?
I only use aluminum spinner and some of the starter cones (hard ones) don't stick well on the aluminum spinners and slip on it leaving nice shiny circle on the spinner. Which was the reason to use soft cone. However, the softcone sometimes stick way too much and stay on the spinner even when I pull out the starter.

It is did not just happen to YS, other 4-stroke engines I had also did sometimes.