I have a Ryobi 31cc bowman ring larger carby rexcel ignition and have not had a problem for years.
Recently it has been playing up and will not idle, it runs fine at high speed but will not idle and slowly dies.
No amount of tuning will get it to idle.
I have put a kit in the walbro, new plug, but it requires 3 turns out on the idle screw to get it to run at high idle but dies when it is set to slow idle.
I have run out of ideas or what could be the answer.

Idle jet channel has to be blocked up somewhere.....if it dies it's a lean mixture.

Is the backplate cracked ? Check the vslve too when you takr the plate off.
Carb and backplate gaskets OK ?

I took it apart and checked the carby again and found the gasket was on the wrong side of the diaphram, that might make a difference.
All gaskets were good, the reed valve was not sealing as I think it should, so swapped it for another, no cracks in back plate.
I am not sure what the "vslve"is. I have not tested it yet, thanks for suggestions.

Oh I think you know what the vslve is.
It's the valve when you are typing at 5am by the light of the screen !
Make sure the carb pulse port is aligned with the gasket and backplate and not partially blocked.