Most gas power pumps, including the Honda WB20, will have a throttle control however, we and the manufacturers do NOT recommend controlling your water flow by the throttle. Instead, it is better on the pump and engine if you install a ball valve on the discharge side and use this for your flow control. As long as you are not reducing your flow more than 90%-100%, you will not cause and harm or damage.
If you have any other questions, please let me know.