Woods Belly Mower Belt ???

I have a 1971 Mitsubishi R2500 with a 60-inch Woods Belly Mower, three blades.
I bought a brand-new 204" belt (I think I remember the length) last summer.
I threaded and tensioned the belt properly and mowed the place half-a-dozen times with never a speck of trouble.
I left everything as-is and parked it for the winter.

I went out there a while ago, fired it up, put the PTO in gear, and the belt instantly derailed.
I investigated everything and could not find a problem anywhere.
I re-threaded the belt and tried again --- and again --- and again.
I got the entire belt out in the sunshine and examined it closely; I could not see any reason for it not to be as good as brand-new.
However, when I get the belt installed and rotate the big pulley that is on the PTO, no matter which direction I turn it, the belt almost instantly starts turning sideways in the pulley.

Any ideas ?
Thanks for reading and all help is appreciated.

Those Woods mowers need to have the belt wrapped and twisted EXACTLY as Woods says. They look right other ways, but won't work. The belt also needs to be very tight, what you think is TOO tight. 

Brian F(IL) wrote: On my CA with Woods L-306, I usually tighten the belt with the mower raised.  The belt will tighten even further when you lower the deck.  It may just be the geometry for my set-up, but it works.

Thanks for sharing that !
I have always just tightened the belt with the deck flat on the ground and had never given a thought to doing it with the deck raised; reason being, in order to be able to thread the belt around everything, the deck has to be as low as it can get and I always just went ahead with my tightening with no thought to raising the deck.
Just sitting here envisioning it in my head, I believe my belt would loosen when the deck comes up and tighten when it goes down.
I am definitely going to give it a try.

I never seen a spring loaded idler on one.

Nope, no spring loaded on mine either.  On BOTH of em.  I've had the belt on my L306 for about 10 years or so now and it's just fine.  It is TIGHT!  I used a railroad bar to pry with and made it TIGHT.  I've never loosened them during the winter months and never had one turn on me yet.  (knock on wood).  I had one break on me when I first got the deck, but that was an old belt.
They can't make a belt with a twist already in it, as there are TWO twists in it when you put it on.
Good luck and remember, TIGHT.

I let the thing - and myself - rest for a few days before fooling with it again.
The grass is now about belt-buckle high so I made one last attempt at getting it to mow.
I drug the belt completely out from under there and made sure that I flipped it over so as to be running in the opposite direction.
It did not like being turned over and kept trying to revert to it's original orientation, but I managed to keep it flipped while I wrapped it around all the pulleys.
Once I got it around everything it was supposed to be around and not around anything it was not supposed to be, I muscled it back onto the big PTO pulley.
At this point, the adjusters had not been messed with; it was at the same tightness that it had always been.
Before today, whenever I hand-turned the big pulley in either direction, the belt instantly layed over sideways in the pulley and started to climb over the edge.
Now, after my flipping the belt, I could turn it several revolutions in either direction and it stayed right where it was supposed to.
I already started to feel better.
I raised the deck and tightened the belt as much as my big long crow-bar and the purchase I had would allow; I gained about 3/4-inch farther down the slot than it had been.

I fired it up and put the PTO in gear, fully expecting to see the belt roll off again --- it did not...
I throttled the engine up to maximum RPM and still doing good.
I put her into that belly-high grass and it was cutting like a new pair of scissors.
The grass got really thick and almost choked out the 2-cylinder diesel, but the belt did not derail.
At this point, I was feeling pretty good about the whole situation and then everything under me got real quiet.
I looked down and there was the broken end of the belt.

Oh well; I am used to frustrations and disappointments, and I have better things to do than mow stupid grass anyway.
It gets on my neighbors' nerves a lot more than it does mine, but not yet enough that any of them mow it for me.
I think I will see if I can find that old decrepit walker that used to be around here and make a big episode about using it to go to the mailbox; then it will look like I have good reason to let the grass grow.

If it gets to be too much of a problem, I do have two big jugs of generic Round-UP and one of 2-4-D; one trip around everything with the 12-foot boom-spray and problem solved.

And the grass was tooo high 

Yep, the grass was WAY TOO HIGH!!!
These are finish mowers, not a bush hog or anything like that.

Did you use a Kevlar belt?  Here's what I use....
https://www.vbeltsupply.com/b245k-wrapped-kevlar-belt.html

When I bought it 7 years ago, I paid $45.91 now it's $68.01.  Quite a raise in 7 years...

Dave, there is something TERRIBLY WRONG with your URL above.  Especially when I clicked on it and it came up with this.......

http://www.https.com//www.vbeltguys.com/

This is the REAL URL for the vbelt guys

https://www.vbeltguys.com/

And besides, they don't have any of the Blue Kevlar Belts aka  B245K Blue Kevlar Belt

Rust or pitting on a pulley will tend to walk a belt off