Why does your yardman riding lawn mowers belts squeal and engine shuts off when blades are engaged?

More than likely you either have a bearing seized up in one of the deck spindles, a bent blade that is in a bind or debris on the underside of the deck keeping the blades from turning. If this is the first time it has been used in a long time, with the mower off, work the blades back and forth by hand until they move freely then start the mower and try to engage them. If they do not start to move freely then you will have to figure out which one is not turning and take it apart.