OK Chaps, here's a bit of hands on feed back.
Replaced the YS91FZ the a brand new OS 91FX and ran 2 liters through it, wet rich.

Placed the tank back on the CG, hooked up the Cline using exactly the same plumbing/valve as with the YS.
Ran very well, seemed to be reasonable dialed in.
The problems started when I began positioning the aircraft (hand held) at different attitudes/different throttle setting.
I was getting quite different mixture responses (from rich to lean) as I moved the aircraft up and down upright and inverted.
Not much mind you but enough to be concerned about.
On closer inspection I noticed air bubble in the fuel line from the Cline to the remote then to the carb.
I'd seal and glued on the nipples but still, air was entering somewhere.

Next I flipped around the plumbing and hooked up the Perry. It took less than 30 secs.
Slobbering rich as some of you had mentioned in the above replies, so I screwed in the low seed NV about 2 turns and BANG.... it was just lock in.
Picked up the plane, moved it up and down, upright and inverted and many different throttle settings... gently and violently... NOT a burble, not a cough.
Absolutely perfect where ever I positioned the plane.
Very satisfied.
There'll be some work to do as time progress and the engine beds in some more but conclusion so far...
The Perry is quite wonderful... rock solid.

Just as a side note, I haven't used a 2 stroke for many years, stayed with the YS's... big and small.
But the FX is so lovely to use. So simple, no nonsense piece of hobby motor compared to the YS.
I mean it brand new... and a quick finger bounce against the compression and it's idling...
non of the pumping up the pressure, turning it over with the starter, heating the glow and waiting for it to fire nonsense!
And quiet with the standard muffler, least as quiet as the YS with the Hatori.
And the power? I'd guess by feeling the vertical pull... more than the YS.
No brainer engine on cheap fuel.
Exactly what I was looking for.
Thanks for your help.

PS I've post a prop q on the forum, maybe somebody can help with that?