Does the carb look like this?:
The vertical screw is the idle mixture screw and in this case you turn it anti-clockwise to richen.
In most other slide-type carbs the screw is fitted horizontally near the idle speed screw, and they work the opposite way, anti-clockwise to weaken the idle mixture.
The rule of thumb is that if the idle mixture screw is on the air filter side, it richens clockwise. This is because it works by controlling the air to the idle jet. The more air you let in by unscrewing it, the leaner the idle mixture becomes. If you screw it in you cut off air to the idle circuit making it richer.
On the other hand, if the idle screw is on the engine side, the screw acts like a petrol tap or valve. Unscrewing it lets more gas through and richens the mixture.
CV (Constant vacuum) carbs invariably have the idle screw on the engine side, sometimes underneath and sometimes on the top, and are turned out anticlockwise to richen the mixture.