Juki dnu-1541 S hard to turn

https://www.manualslib.com/manual/814520/Juki-Dnu-1541.html

https://youtu.be/aaNsmgHQzTo

https://www.youtube.com/watch?v=6PGPtkjsqus

Here is an online resource for the manula:
JUKI DNU 1541 Instruction Manual

It's on page 21 of the owner's manual (download available here):

Juki DNU 1541

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Here:
JUKI DNU 1541S Manuals

https://www.manualslib.com/j/juki+dnu-1541s.html

Your bobbin system isn't engaged. this can happen for several reasons, mostly when things are a little out of whack.
The 1541 has a 'clutch' it located near center on the bottom surface and is a silver circle below the 'arm'. Bring the needle all the way down, now push the clutch and hold while you turn the wheel back (top away from you) eventually the clutch will click when everythings aligned and engage the bobbin system again.

Go to http://www.dixiesewingmachine.tzo.com/SERVICE%20MANUAL.htm they have a copy of the engineer manual for this machine in a pdf file and all the info you want on timing is in it.

go to www.juki.com.jp login industrial sewing machine and you can doenload manual for juki machine

Might I be so bold as to disagree with Allmakes.
Why would a manufacturer build different models of industrial. They are designed for differnt jobs. The 8300 is a high speed garment machine, the 1541 is a compund feed upholstery machine. The motor is the same HP and does not affect the ability to sew heavy. The garment machine will sew garment weight leather just fine but you will have problems getting the fabric to feed properly particularly over seams. The other issure you may find with a garment machine is it's ability to handle heavier threads. I suggest that the 1541 would be a better choice for making bags and purses.