I've never understood what was meant by "tuning" the mechanical advance system.

Sure--you can advance the timing in the middle of the RPM range by adjusting the rod length.

But, then you've screwed up the timing at all the other spots in the curve.[]

For example:
Your idleing at 6* BTDC and your at max advance--or 28*--at 5500RPM. Everything from 5500RPM and up to max RPM is at 28*. And thats fine. Maybe you don't need a bunch more advance from that point on.

But--lets say you want to increase or decrease the timing in the middle--around 3800RPM. Maybe the advance is currently set at 14* at 3800RPM and you want to set it up to 16* for better throttle transition. So, you lengthen or shorten the rod to gain some advance in the middle.

Haven't you just screwed up the idle and top end timing????