If your Chetak runs fine do not mess with the air mixture screw on the carburetor. This is the other screw, that does not have a spring on it (that one is the idle screw). The air mix screw is set at the factory. Should you somehow end up messing with it, to find the proper setting first start with the screw bottomed out and then count complete revolutions as you back the screw out. For my scoot, I found 1.5 turns, too rich (resulting in a dry black spark plug) and 2.0 turns was too lean. For me, 1.8-1.9 turns was just right.