I changed the belt on my Grarely Pro 150 and I installed the belt as diagramed. But now the blades still run when in neutral. I don't think it's the new belt because the drive gear runs even with the belt off. What could be wrong?

I have had this problem on a riding mower before. It ended up being that one of my pulleys on the deck was loose and it was meant to cause some drag on the bely. I replaced it with a new one and my blades stopped moving.if you can do it safely with gloves try putting resistance on the belt without forcing it onto the pulleys.(grab the belt in a long stretch with gloves slowly)