50/50 . though all castors fine . Tom Dixon regards castor as doubling the ring life .

Generall concensus is 18 % total , or a stutter / stagger from residue in the backplate peripharyacumulates and clears causeing this .

fine on no nitro , maybe even best . Though a prime of ' nitro fuel ' in cold weather will wake it up easier .

5 or 10 % Nitro . or 3 to 10  % . Maybe  extra gasket or two . But Dixon'd be a Hemi'd head , so likely unnessesary .

Theyre actually vary durable . Ran a good ten gallons of all sorts  through one . Including one of stale sig ' F A I  juice . Maybe double that.
Recent strip & clean , the ring was at a tight 5 thou. gap . An Italian version .

ADDED : ( see next post ) needs a few gallons through it before its ' loosened off ' a bit . !

They run good on the silent muffler ( as does a Merco 49 ) if youve the room & need noseweight . lots of power .

For setting , theres 3/4 turn , solid 4 to solid 2 . Wind it into the solid 2 slowly , then back 1/4 turn , usually gets ' the good ' setting .
Just going in from the 4 , your usually cautious and end up short / rich . Too Far .Get it into a steady clean two stroke, then back it off into a even gnarly 1/2 n 1/2 run.

a good tank 7&itll just switch through at 45 degrees , opening it out a tad and you can get it just into the switch across the top of the circle .Of if its a gale , in till
its pulling say halfway up the loops . Should cut in & out dead on the same hight. or try another tank . Usually use baffled wedge corner uniflow .

Maybe Enya 3 for cold fuel . ST plugs seems good for all fuel . maybe enya 4 with nitro .
The little std 46 muffler , with the tailpipe cut off is a good muffler for them . You can butcher the fins and nose a bit for streamlining .

Further to . . . Here in Aus, Prefered cool power , Syn & Castor . 50/50 , maybe 18 % winter , 20 % summer . Nitro accoring to taste , unnessesary , really . But likely a small syringe
to meter ' nitro fuel ' prime , if cold weather or competition . Sometimes hot weather needs it, you see Urtnowski hitting them with lighter fluid instead , for the same reason .

Hi  Use  23 %  oil   with  half synthetic, The ring will last longer  on that brew  than  with 18%  or  all castor   Hint  18%  is  not enough oil  for that to cycle  correctly,
100s  of  people  use this  brew in their  ST51s   and  many other  ST ring engines
Randy