Changing the input rate of a hopper

I have a model of a hopper feeding particles into a box where the radius of the hopper outlet is 0.006m.
I want to decrease the rate at which the particles are fed from the hopper into the box. I have tried to decrease the size of the outlet to do this but it just jams the outlet so no particles enter the box.
Is there a way to set the input rate from the hopper to box?

You need to model the box only, and use a mass inlet boundary condition to specify the flow rate.