I just changed the main drive belt in my 2 year old D125 and put on new pulley's as well. I ordered the parts from John Deere (GX20006KIT1) so I didn't have any issues. I noticed the old belt was pretty tight and when I put on the new one there was a lot of slack. I compared the belt size and pulley sizes and they are the same. I made a quick video as a visual. I know it's something dumb I'm doing but not sure what. Can someone tell me what I did wrong?