sounds to me like your using your timming to compensate for what tuning your lacking in your carberation. If you are accurattly set at 28 deg and it misfires or what has been reffered to as four stroking ( cause it sounds like a four stroke motor) then it has to be that your carb is not set right or you have a carb problem.

Also I went to spad fest saturday. man what a cool event. Everyone said I had the best running ryobi they have ever seen. Looking at some of the other planes i really want to do a quad spad with ryobi engines. I think i got next winters project lol.