Feel free to experiment, but these little engines aren't very sensitive to timing. 28 degrees is very close to optimum for most.

If your regulator diaphragm on the carb is exposed to the oncoming airflow, it will often make the engine go rich in flight. In cowled installations, I solder a brass tube into the little hole on the diaphragm cover and route a hose from it to an area of non-pressurized/disturbed air, such as into the fuselage. On my non-cowled engines, I usually just solder a 90 degree piece of brass tubing into the cover, pointed anywhere away from the oncoming airstream.

This usually isn't a problem with the rear mounted carb on the Ryobi, but maybe it is somehow getting some pressurized air to it in flight.

AV8TOR