Adjust the high speed needle valve (big one) until you get maximum RPM with the throttle wide open then richen it (open the needle) slightly so you get a 200-300 RPM drop. You can do this by ear but a tachometer really makes things easy & accurate.
The low speed needle could be either a fuel control type or an air bleed type depending on the engine/carburetor. Regardless, turn it 1/8-1/4 turn at a time one way or the other. Open the throttle to clear it out then bring it back to idle. If it seems worse, you went the wrong way. Adjust this needle until you get a smooth reliable idle and a good throttle response without stumbling.
You can also do a "pinch test" to determine what the engine needs. Pinch off the fuel line going to the carburetor. If the engine speeds up noticeably then quits then the mixture is probably on the rich side. If it just immediately sputters & quits then the engine is likely too lean. If RPM slowly falls off with possibly a slight rise when you first pinch the line then the engine quits, it is likely fine.
This inexpensive tachometer works pretty well:
http://www3.towerhobbies.com/cgi-bin...&I=LXPT31&P=ML