My Mom has an MTD 42" riding lawnmower. The mower starts just fine, but you engage the blades the mower either goes dead immediately or runs very sluggish. Could this be a shear pin problem or something binding under the mower deck? Any help would be greatly appreciated.

Pull the spark plug wires, disengage the blades and manually turn. Do the blades spin freely?

I'm don't know if they'll turn freely or not. I don't have the mower at this time. I was going to pick it up today and drop off my rider, but we've got rain again today and I haven't gotten it yet. Thanks for your quick response though. I'll check that as soon as I get to the mower.