One of the problems with some fuel pressure systems is that they maintain a fixed pressure with little change while the FF actually changes significantly.  If the system is a fuel pressure system of gauge display, you will not see the FF change that is actually happening.  If you have a FF transducer in one of those systems, you can see this by the differences between the two instruments.

This is one thing I do not like about the TCM FADEC.  It displays a fuel pressure which does not change significantly with rather large changes in FF.

In many carbureted installations, the fuel flow instrument is actually fuel pressure and does not show the FF changes accurately.  I am not sure how the Mooney system is set up.  I do have a great deal of confidence in telling you that as you reduce throttle from WOT, the first 3/4 to 1 inch of throttle travel does not reduce MP -- it reduces FF only.  This is the very important fuel enrichment feature that is built into almost all carburetors.  A very, very few do not have this, and they are almost always found on turbo-supercharged engines like the big radials.

Walter Atkinson