Hello!
The OS LA engine you have is an excellent engine ( I have helped many newcomers in our club having this series of engine and all have worked perfectly from the start needing only slight idle adjustments.
The good thing for newcomers using this series of engines is that the carb with it's air-bleed function makes idle adjustments nearly unnecessary. Idle is nearly perfect from the start.... The idle function using the air-bleed screw is very marginal...so newcomers cannot really go wrong here.
But if you still can't make you engine behave properly you most have done something wrong!
Begin checking all the bolts on the engine. Also check the glow plug copper gasket...this gasket should not be reused when you take away the glow plug...always put a new copper gasket in whenever you change glow plug!

Fuel: Nearly every engine on the market runs better using some nitro....OS engines like 5-15% nitro ......more and you have too ad a (or more)0.1mm head-shim to reduce compression.
5% nitro is what I use in most of my engines.

Oil: Over here in Europe it is very popular to run 10-15% all synthetic oil like the French Motul "Micro" or the German Aerosynt 2 or Aerosave...but 20% castor oil is of course also working in your engine.

Glow plug: Always use a Enya 3 or OS 8 glowplug.
Propeller: If you fly at sea-level and it's a high-winged trainer you have ...use a 11x6 APC prop....10x6 will work too but the larger diameter of the 11" prop will give a more safer and lower idle (Idle should be around 2000rpm with the 11x6 APC).

Carb: The OS LA carbs have an air bleed screw that adjusts the incoming air through a small hole up at the front of the carb ..you should adjust the idle screw so that the hole is only slightly closed by the screw (look closely and you will see the screw treads inside the hole): most of the hole should be open....the screw should only be seen slightly when you look inside the hole. This idle function is not that sensitive and could for most of the time be left alone as it is......
No drilling up of the idle hole in the carb is necessary!!!
When you set the cab-drum opening it should be 1.5-2mm open in idle position
At this position the engine should idle at around 2000-2200rpm all day! and react instantly when you give full power.....

Fuel tank: It is of vital importance to most models that the fuel tank is not too large.....for a .40 -.46 engine a 8oz-10oz (240cc.300cc) fuel tank is perfect!
Larger tank ...and you might have slightly more problems adjusting the engine.
It is also very important that the tank is positioned so that it is close to the engine and mounted with lots of foam around it. Vibrations could other vise disturb fuel distribution.
The centre of the tank should be, with the airplane sitting level, inline with the fuel-inlet orrifice in the carb.....this is very important.
No air-bubbles should be seen in the fuel hose going from the tank to the carb when the engine runs.
Fuel hose should be made of silicon. 1,5-2mm in inner diameter....not larger!
And last....You have to listen and learn how your engine sounds when you do the tweaking on the carb main needle...this is how one sets an engine....by ear!

Hope you get your engine going.
Regards!
Jan K
Sweden