Heidelberg Windmill - flywheel belt coming off

Generally starting in the fast speed OR the slower speeds, makes no difference to the belt *Running ON* true.!
That is governed by the >CROWNED< Flywheel, which induces the belt to Run On all the time.
If You take a close look at the Variable Speed Driving Pulley,! You should observe that across the entire speed range, the fixed Disc of the *V* maintains the same attitude and approach of the Belt To the flywheel, the >SPRUNG< Disc, backs of but still keeps the Belt Running True, so fast or slow starting (should) make no difference.

2 possible causes for the above conundrum:-
A. The motor on its mount is out of line, via the transverse shaft it sits on, and swings on, (axis wise) for adjustment.
Adjustment check, via a Plumb LIne across the (outer) face of the Flywheel and the rear flat face of the fixed *V* fixed Disc, - line of sight, rule of thumb, basically.??

OR
B.Unfortunately the Belt has reached its *sell by date* and some of the Cords within the Laminate, (can be seen within the *v* of the belt) cords parted on one side, within the laminate, causing distortion and causing the Run OFF.

Can be checked thus: turn the M/c, over by hand (trying not to drop the belt if possible) wind the speed control UP till the Variable speed Coned pulley, has allowed the belt to reach the bottom, of the Cones, insert a baulk of timber into the bottom of the splayed Cones, (to keep them in the max., open gap) wind the speed control back (or vice versa!!) to release the belt with the option of replacing without too much trouble, - baulk of timber etc., - Without removing completely, inspection from above (Plan View) should show either reasonably straight, OR seriously BANANA/BOOMERANG syndrome, implying shot cords within the laminate.

Absolute worst scenario, apologies, hopefully only motor out of line.
.

The first thing I would check is the belt adjustment.
The manual:

https://letterpresscommons.com/original-press-manuals/

gives good instructions on how to set up the belt properly. When I first set up my Windmill I had a number of difficulties with the belt and spent quite a bit of time getting it set up properly. The belt changes position on the flywheel as the operating speed of the press is changed. So, if not properly adjusted I could see the possibility of it slipping off.
Good luck…

Thank you so very much Mick on Monotype and dwallen! Can’t tell you how much I appreciate the response. I’m going to start by checking the motor to make sure it is in line and hopefully that solves the problem!

Again, thanks so much for all the help!