Sounds like you may have turned the low speed needle in a little too far. Try backing it out a turn and see what it does. You want to adjust it for a reliable idle and a good transition from low rpm to high rpm.

there should be instructions for adjusting it in your paperwork.

Good luck
Tim

Dual needle carbs such as that engine has, the idle needle can take over high speed control. When that nappens, nothing will work correctly.

Basic setting for the idle needle:
Back it out 5 or 6 turns.
Close the main carb barrel (servo control)
Carefully turn n the idle needle until it either has resistance or moves the carb barrel (watch carefully)
Open the idle needle 2 turns. That will be a sloppy rich idle.

Close the main nedle and open it to 2 1/2 turns.

NOW, start the engine at 1/3 to 2/3 throttle.
Adjust high speed with the main needle. DON'T touch the idle needle unless the high speed is set.

Slowly reduce throttle and adjust the ile needle very gradually. 2 clicks (*1/8 turn) at a time maximum. You are looking for the engine to run smoothly at as low a speed as pssible. The best adjustment will have a stable idle with the carb barely cracked open. (main barrel) The engine should be able to sit at idle for 20 sec, then you slap the throttle to full and it will hesitate less than 1/2 sec before responding.

The low idle adjustment on an os (and most engines) are pre-set....They are set right most of the time.......(altitude and temp are varibles).....Most of us never mess with it after we establish a good run after break-in.
Rule of thumb.....don't mess with low idle screw unless you know what you are doing......
It is Ying and Yang with the top end rpm, and mid range.
Move one....requires a tweak on the other......
Go back to square one as per instructions provided....Start all over...
Sounds like your engine is fat with too much fuel.
Doc

Turn in your high end needle all the way closed and take the fuel line off and put a new piece of tubing on this line. Blow into this fuel line and back out your low end needle just until you hear air coming from the opening of the carb. Then set your high end needle out 2 or 2 & 1/2 turns. Crank your engine with the carb barrel just slightly open. Always make sure that the radio setting for starting your engine for idle on the slide tab only allows the engine carb open just slightly.
To properly set your high end adjustment after you get the engine running, have someone hold the transmitter and you hold the plane, turn the throttle setting all the way up if it will take and with the plane on the ground. Take and pinch your fuel line and listen to the engine. If the engine starts to rev up then leave the high end setting alone. If the engine sputters then you are too lean, back the high end out a quarter turn or so. After doing this run the engine back to full throttle again and this time hold the plane up so that the nose is pointed to the air and listen to the engine.
Now we need to set the low end needle setting, make sure your throttle stick is set to idle. Take and pinch the fuel line to the carb and listen. If the engine starts to rev up then you are alright, if it stumbles then you are too lean.
Reverify high and low adjustment prior to putting her airborne.