Hi!
It sounds as if you run your engine too lean! Just open the highspeedneedle somewhat and try again!
But........Here is what recommend for you OS LA .40.
First ..you dont have to use more nitro than 5% to have this engine perform at its best! 15% nitro is max!
The propeller is vital....use a 11x6 APC for relaxed and slowflights with good climbing power (if you fly at sea-level).
A 10x6 will produce more explosive power and this is not recommended for a newcomer...
Fuel...well I use 15% syntetic oil like most people in Europe...but "good ole" 20% castor oil will work too.
Fueltank size is important ...a 240cc (8oz) is perfexct for your engineand every other .40 engine.. see too that you have the tank well surounded by lots of foam padding and that the tank is placed with the center line inline with the carb orifice when the airplane is hold horizontal....or 1cm below the centerline! This is very important!
Naturally you use pressure from the silencer to the tank!
A good glowplug to use is the OS 8 or the Enya 3.
And do not use sheap plastic spinners!
If you are going to use a spinner use a Tru Turn aluminum spinner! I myself prefere aluminum spinner nuts on my sport airplanes , easier to start engines that way .....only turn the spinner cone on the starter inside-out!
Setting an engine is something you have to learn...the pinch test might work ...but I dont use it ...A better way to set any engine is by ear....you listen to how the engine sounds and take appropriate action....
Your OS LA .40 has an airbleed carb which will work as good as any other two-needle carb...and it doesn't have to be modified at all to function perfect! Just listen to how the engine sounds and act accordingly...allways setting the highspeed a tad rich when holding the nose of the plane straight up! This is the test you should do every time you fly your airplane!

Good luck!
Jan Karlsson
Sweden