I don't know... I have thought the SAME damn thing though.. I know they just alter their timing on the flywheel or key and just advance where they want it set for on the racing engines. I have pondered that one myself many a time.. 
All you'd have to do is build a bracket that would allow your coil to turn around the flywheel some like the boat motor ones do... getting the ignition curb and stuff right for the rpms and the cam and stuff would be the bitch of it all. :doah: You'd almost have to use the governor or something to tell your coil where to be to match the RPM's though.. Who knows? A lot of building for what it is. :doah: Not like I haven't done it before though I guess... :doah: lol
All you'd have to do is build a bracket that would allow your coil to turn around the flywheel some like the boat motor ones do... getting the ignition curb and stuff right for the rpms and the cam and stuff would be the bitch of it all. :doah: You'd almost have to use the governor or something to tell your coil where to be to match the RPM's though.. Who knows? A lot of building for what it is. :doah: Not like I haven't done it before though I guess... :doah: lol
Honda Trail 70s with the 3 spd engines did not have timing advance either maybe a few other models. I,m wonder if they thought it might more reliable with out timing advance. I would guess that TRK maybe right, the engines we use on minibikes were only design to maintain 3600 rpm original so they did not see a need for it.
1.) They are designed to spend the majority of their useful life at a steady 3600rpm with WOT. Advance curves are for engines that will see a lot of use in transient conditions like below redline or throttle variations. Even they can function fine with "locked" timing, but response is better when the timing can somewhat compensate for load.
2.) Much to their credit, Kohlers were built like single-cylinder automotive engines of the time. as a result, they were better than Briggs or Tecumseh(with a price to match) and I'm not at all surprised they had centrifugal spark advance.
In the end, yes, a nonlinear spark advance is beneficial. With locked timing, you can really only aim for the right amount of advance in a narrow range of RPM. Ideally the ignition controller would be programmable in relation to RPM and manifold pressure or throttle position.
2.) Much to their credit, Kohlers were built like single-cylinder automotive engines of the time. as a result, they were better than Briggs or Tecumseh(with a price to match) and I'm not at all surprised they had centrifugal spark advance.
In the end, yes, a nonlinear spark advance is beneficial. With locked timing, you can really only aim for the right amount of advance in a narrow range of RPM. Ideally the ignition controller would be programmable in relation to RPM and manifold pressure or throttle position.