RE: another darn ryobi post (ryobi performance modification, analysis and comparison)
Even though the carbs above are obsolete - repair parts are still available
The original Walbro and Zama carbs on the Ryobi, Ryan, and Lawnboy 31's was a "20" size
They run a lot better with a "24" and a "28" is about as big as you want to go.
I like the WA-167 carb for it is a "28" and has the high speed check valve but no accelerator pump. It was the Carb for the Zenoah G23 just before they came out with the G26.
Without a lot of porting and running high rpm, a carb bigger than a "28" doesn't run well. On the Ryobi, you start running it at 8000 and the stamped steel rod breaks.