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P3D v5 How to slow a prop...

Ok... I've got a turboprop... everything's looking decent... except one thing: when I pull the throttle back... the propeller RPMs don't reduce until the throttle is almost at idle. Ng drops back as expected based on throttle position... but Np doesn't change until throttle is almost idle... and even that's not instant... it's delayed. It's as if the prop just want go and go and go and..

So, the question is... what value can I touch (NOT friction) that will cause the prop to slow down when it doesn't have higher Ng values to drive it? Is it propeller MOI? Perhaps it's too high and thus the prop just keeps going and going and??

Thanks in advance...
Try "Turboprop Engine Tuning" 28 July 2018. Several good entries that discuss the subject of turboprop engine control
Actually that is of very little help.

What is happening is that in flight, the aircraft flies along at throttle... everything appears to be fine. You pull the throttle back... and N1 drops as expected... but N2 does not change at all until throttle is at idle. That's not correct and I'm asking if there's something that specifically impacts that particular behavior.
It is simple enough to accurately model a straight jet engine, because the sim has an excellent model built in. However the turboprop model is pretty bad and most developers will resort to their own external models.
If you stay with the in-built model you have two major factors, the engine tables, 1503, 1504, 1506, 1507 and 1508 and the propellor tables 511 and 512.
From what little data you have given I would guess that your problems lie in the propellor tables. They essentially govern the prop RPM. There is no simple answer. Each case is different and it took me more than 3 months continuous work to get Dino's C2 and E2 engines to produce results close to the correct values. I did not use an external model because Dino asked me not to, and there was a mass of test data to work through.
Good Luck