What size carburetor do I use on a 50cc gy6 motor 19mm or 20mm?

For a 50cc GY6 motor, it is usually recommended to use a 19mm carburetor. This size carburetor will provide sufficient airflow for the engine to operate properly and efficiently. A 20mm carburetor may also be used, but it may be too large for the engine and could result in excessive fuel consumption and decreased power.