Engine idle speed may be unstable for several factors. Everything is determined by the vehicle brand. The reason of the broken engine performance during idling is not only in the carburetor, but also in the system, affecting the value of the vacuum motor intake of manifold. So before you buy a good car it is better to check engine operation.
If jets of carburetor or system channels are clogged, the motor speed may fall drastically and short. Operation of the transition system will occur at small valve angle of rotation, and during the further opening of the throttle fuel immediately starts to fall into the second chamber, passing the transitional system. To eliminate this drawback make the washing of the carburetor. In case of sudden discharge of gas, there is a drop in speed until the engine is completely deaf, than you need to repair system of idling.