Tuning For Detonation

Now it's 35 at 3000 RPM with 20 initial.

With the mechanical advance, turning the spring perch for the long lopp spring will bring into to play at lower rpm.

maybe this will help complete the picture.
Lets say the advance looks like the dotted line; And lets say you want to keep the 20* initial and 35* max.
upload_2021-4-30_12-43-19.png

Turning the long loop spring's perch or using a shorter loop will change the advance like this.
upload_2021-4-30_12-54-1.png
It only takes a few degrees out of the mid rpms and leaves the top and low rpm timing as is.