Once I got the engine running decently, with the new cylinder, proved out what Rcpilet said about the carb/prop thing. The larger carb did little for the large prop, but made quite a difference for the higher reving smaller prop. Had already made a new crankcase back to improve the "stuffing" show. That's the easy part. Have to trim the bottom of the cylinder, & the rear of the piston, to clear the new cover. Also have to design/fab a new reed/manifold assembly, to work with the new cover. Two or three protypes made out of wood, then try to make the real thing.
Agree with AV8TOR, that 150 deg on the exhaust, would probably be a goog place for the low reving Ryobi. However, while going up in 5 deg increments from the original 118, found myself at 155deg, with no improvement. Only them did I think about the possibility of the intake ports also being misplaced on the "bad" cylinder. Checked them, & sure enough only had 85 deg! I probably gave the cylinder the final kiss of death by not being more attentive, but will proceed to increase the intake duration, & check things out, before tossing it into the trash. .............Geo.K.