Where can I find the New mod?  Is this a part I can print?

The STLs are available to print - PC-CF is recommended. 

https://www.printables.com/model/531604-mmu3-printable-parts/files

From memory I think you'll need a magnet, an o-ring, and a couple of different length screws for the idler.

Instructions here.

Posted by: @alienbliep

But it doesn't look like your nextruder is modified for mmu. (sensor + gear case)

Good spot!

Where can I find the New mod?  Is this a part I can print?

Regarding the tension springs, I think I have tried every possible variation from very loose to quite tight.

These spring tension screws need to be straight with the end of the thread. 

This maybe helps becasue filament does not easy push the sensor to loaded state because springs hold it back.

But it doesn't look like your nextruder is modified for mmu. (sensor + gear case)

old

New

If these numbers are Ok then maybe there is noise on the filament sensor, possibly overshoot or some other dynamic of the signal that makes it trigger too soon?

Procedure:

Have a USB stick inserted.  Turn on Settings | System | Save Logs to File.  Run the calibration.  Turn off Settings | System | Save Logs to File.  Eject the USB stick, put it into a laptop and locate the logs.txt file.

3543.259s Started logging to file: /usb/log.txt

3572.911s Started

3576.073s user pressed continue

3587.831s calibrating

3587.952s Calibrating NoFilament value: 1788786

3589.911s calibrated

3589.911s calibrated

3603.747s insertion check has filament

3608.761s user confirmed filament is inserted

3608.811s calibrating with filament inserted

3608.891s Calibrating HasFilament: PASS value: 561346

What I see is that during Stock filament calibration the extruder motor is freewheeling and I can insert the filament all the way to the hot end.

During MMU filament calibration the extruder motor is controlled.  When the screen asks you to insert the filament, the extruder motor is running and the gear is turning.  But the motor and gear stop when the filament is just barely inside the pinch.  This is where it collects data on what is the value of the sensor to indicate "fully inserted".  Since the motor is controlled, you cannot push the fiber any further into the hot end.  The picture shows where the filament sits at that time.  

Looking at the Info | Sensor Info | Filament Sensor we can see that with no filament the sensor is around 1800000, when the filament first touches the sensor and is not yet in the pinch we see 572000, and when it is fully loaded into the hot end 254000.  

Is there any way to see what values MMU Filament Sensor Calibration uses?  Despite many attempts, I still see the filament stop much too soon and not get picked up by the pinch.

My suspicion is the calibrated value is too high causing the sensor to trigger before the filament is inserted enough, like the picture in the original post.

Position of filament loaded during MMU calibration

Settings -> Hardware -> Printhead -> Nextruder

The ramming is controlled in the filament settings in Prusaslicer.  In the Advanced section there is a button called 'Ramming settings...' that opens up a configuration window.  I had some success in making the rammed tips of my ABS filaments more (totally!) reliable for unloading and loading by reducing the ramming time from the default 1.25s to 0.5s.

Bootloader 2.4.1

MMU 3.0.3

Buddy 47