Why my Yamaha SZ run smooth at low speed?

It is possible that your Yamaha SZ is not running smoothly at low speed due to a number of different factors. 

The most common cause is a dirty or worn spark plug, which can cause the bike to run rough at low speeds. 

Another thing to look at is the air filter, which if clogged can reduce the airflow to the engine and cause it to run rough. 

Lastly, the fuel level could be too low, resulting in the bike running rough at low speeds. If none of these seem to be the issue, it may be necessary to take it to a qualified technician for further diagnosis.