... FBO Systems sells a limiter plate that looks like it will change the advance mapping to allow less initial timing, if needed. If I'm understanding you correctly, this might make the idle easier to tune.
Maybe, depending on where the slot is positioned. It will probably mess up the rest of the curve because Don (and most people) use the slots as hard stop for the max advance.
You could try buying one and modifying one set of slots so they are shorter on the inside. This will shorten the advance and put more initial tension on the primary spring; And it won't mess with the secondary spring controlling the max.
It's been a long time since I've taken my tach drive distributor apart. I think the advance slots are slightly angled. If so, make sure the slots in the FBO plate are wide enough to accomodate that.
If later on you need to adjust when the secondary spring engages, the spring perches are on eccentrics that can be rotated with a screwdriver.
