Hallo!
It is my experience from 27 years of R/C flying that if an engines behave as you discribe itīs too lean......
But to be sure ...how does ist behave when you advance the throttle...?? does it sputter and hesitate...before it rews up ?? Then itīs too rich!
On the other hand if it just hesitates for a 10th of a second before it rews up or if it just dies when you give full throttle itīs set too lean and you have to open up the idle needle, (counter clockwise).
When discussing idle rpm, 2000-2300rpm is a good idle on any two or fourstroke engine. Any lower and it gets hard to maintain a safe idle.

Some adwise:
The carb opening should be around 1-2mm open at idle!
Fuel: 5-15% nitro
Oil: 15% Syntetic, Motul "Micro", Graupner Aerosynt 2 or Aerosave. ( this is what we use in Europe on all engines, Castor is a thing of the past).Motul oil could be bought in motorcycle shops as the French Motul company specializes in making oil for motorcycle racing.
Glowplug: OS F
Fueltank: For a Saito .72 a 200cc-240cc (8oz)is enough.The tank center should be in line with the carb fuelinlet orrifice when the airplane is horizontal.Tank as close to the engine as possible
Propeller: APC 12x7,12x8 13x6, 14x4

Try it and see for yourself!
Good luck!
Jan K
Sweden