It's one of two things:
1) The governor arm needs to be adjusted - here are instructions for a
honda gx100 the principle is the same.
2) Your governor is malfunctioning internally. The weights may be stuck, the arm that goes through the case may be binding, or some other thing causing it to not return to idle.
Note: it is normal for the butterfly on the carburetor to be in a 3/4 open position until the engine starts but it should snap shut once started. If not it is most likely one of the above.