Hi!
The idle adjustment screw on the OS LA engine is not at all critical.
In could not be your problem!
All (OS) engines just run fine if you provide the right things: Fresh air, Fresh fuel and fire.
It's that simple!
I hope you tightened all screws on the engine and checked fuel so that it fresh. I have 10 year old glow fuel that works just great so I don't mean "New" in that sense... I mean it hasn't been contaminated by drawing moisture from the air, having the fuel cap open long periods is not good, especially in a damp and hot climate.
Glowplug: a Enya 3 or OS 8 glowplug.
Fuel: 5-15% nitro
Propeller: 11x5 or 11x6 APC if used in a high winged trainer...not a 10x6 please!
Tank: 240cc (8oz). Do not use a bigger tank than 300cc (10oz).
High speed needle open 2 turns. This is a standard setting on most engines...adjust it from here.
Al OS LA engines are super reliable and will give instant throttle response direct from the start. Very little running-in is necessary.

Idle needle on this engine is an air-bleed needle, it governs the amount of air coming into the carb at low speed. You can see the hole up front on the carb where the air comes in. factory setting on every OS with air-bleed screw is: hole half closed by screw, adjust it from there, mostly you have to open it some.

Regards!
Jan K
Sweden