The best carburetors are those that have two needles one for the high end and one for the low end. This allows you more control over your engine's performance you can adjust your idle setting to give smooth quick power up and then the high end to give the most power.
To start set the high end about 2 turns out with the engine off, open the carburetor to full throttle setting. Adjust the low end setting so there is about a 1/16th inch gap between the two needles in the center of the carb. Start the engine at about ½ throttle remove the glow plug ignition wire and throttle the engine to full power if the engine sounds bogged down (or wont start lean it out a little) until the RPMs start to come up continue leaning out the engine until you notice a drop in RPMs then slowly back the needle out to fatten the fuel mix back up until peak RPM is obtained. Once the engine is running at peak rpm fatten ( back the needle out) the mix a little more until a slight drop in RPM is noticed this setting should provide the most power because when the plane is flying the mixture will probably lean out slightly. If you launch at peak RPM chances are it will lean out too much and die. When in flight if the engine is running strong then dies you need to make your main (high) needle a little richer. If the plane is running at full throttle but sounds bogged down and there is a lot of smoke you need to lean the main needle.
Once high end is set and before you launch your model set the low end. To do this start to throttle down until the engine sounds like it wants to die. Then fairly quickly throttle back up if it bogs down and lots of exhaust smoke then you need to lean your idle, do so until the engine idles well and throttles up quickly without dying. If you throttle up and the engine dies right away then make your idle richer.
single needle carbs
found mostly on small engines, just refer to high end setting above which still applies, just ignore anything about low end.