Puma 185 Oil flow

Evening all. I've a new to me Puma 185, 2014 powershift with under 2k hours. Bought as a fieldwork tractor. Went to start drilling today (Amazone combination) The hydraulic motor / fan on the drill overspeeds even with the flow on the spool valve turned down to min. Return pipe is into the low pressure return (dumps directly into the rear axle). On the motor there is a by pass valve which I adjusted to reduce the flow into the motor. Unfortunately all this achieved was to froth up the oil until it came out the breather.
Is there another way to reduce the oil flow? Non electric valves so don't have the onboard computer option.
All I can think of is a flow restrictor, washer with a small hole in the supply pipe to the motor.
Many thanks
Kenton

Thanks, will have a look and see if that's the issue. Nice easy fix if it is

Thanks Dave, you were spot on.
For future reference.
Pop off knob face
slacken nut
remove knob
adjust spindle
re fit knob.