RE: Ryobi 31cc Conversion
I bought the same engine and made a prop hub adapter and motor mount for it.
Thought it would be a inexpensive setup, turned out not that great.
With a master airscrew 16x8 black plastic prop, I was able to get approx 5400 to 5700 rpm.
Do not know if this is typical for this engine, just my experience on one.
I have a Tartan twin and 3W150TS for my other gas engines.
It seems like the engines need to be port timed to run in the rpm range we need.
Weed wackers do not need to turn high rpms or have lots of torque, if it loads up, back off the weeds.
My 2 cents, hope this helps.