This type of PTO design has the belt loosening when the PTO arm is off and being drwn tight by the spring on the end of the cable. If the belt loosens too much, it will bounce when the PTO is released and try and jump off a pulley. This is why it is very important to have the belt keeps in place and the covers over the spindle pulleys. They act as keepers too. If your belt is too loose, it will probably jump off. If your belt is too short, the blades may continue to run, at some speed, when the PTO is released. There should be brake pads, about 1" square associated with the spindle pulleys. They come in contact when the PTO is off to hold the blades from turning. Also, with the belt loose, or uninstalled, check all idler pulleys to see if they turn freely and are all in level with each other.
On one machine I went 1" shorter on the belt and on on other, I had to go 1.5" longer. This was after checking everything I mentioned above.
On one machine I went 1" shorter on the belt and on on other, I had to go 1.5" longer. This was after checking everything I mentioned above.