I don't know if this forum is still active but in the past Mitty asked if there was any progress. Anyway here it is:

After many months of neglect finally I returned and finished this project. The hardware is a 31cc short shaft, Ryobi taken out of a Harbor Freight weed eater and fitted with a Wacker Engines muffler, mount and prop adapter. A friend machined the flywheel/magneto down to seat the prop hub (that was a bear of a job). The spark coil and resistor lead is from C&H Electronics and the high voltage CDI module is from Charlie Moore. The automatic advance is via a PIC chip, a 16F628 that is programmable. The PIC reads a Hall switch which senses the passing of a rare earth magnet glued to the prop hub. The PIC then counts internal clock pulses to measure the period between each passing of the magnet. At each passing of the magnet, the PIC starts another countdown to send out a delayed, measured pulse to the CDI module, which in turn sends a pulse to the coil, and finally a spark occurs. The software has a programmed algorithm that computes how long a spark delay is needed for the revolution time period. In effect it relates a spark delay angle to RPM, which is the basis of the computation.

Ok, with that as a background here are the results. On the factory magneto the lowest reliable idle was about 2,800 RPM and full throttle about 6,400 RPM turning a 16x8 inch Master Airscrew prop. (My driveway sits at 5,500 ft. altitude and the 15% thinner air makes for lower power.) With the operational PIC/CDI combination the performance improved dramatically, with easy idling at 1,500 RPM and full throttle of 7,800 RPM. Another benefit is easy hand starting after the engine has warmed up, always on the first flip.

I had plans to do some engine hop-ups such as installing a large throat carb, opening up the read valve stop, increasing the compression ratio, etc but this puppy runs so well I have put all that on a back burner. On to mating this to an airframe and flying this puppy!!