If this helps, I found the manual for my machine. The manual covers models 13565 and 13607, so far not able to figure out which model I have, although I know it has a 12 HP B&S engine. The transmission design doesn't click into gears when you change gear selections. The loose or slipped belt seems like a good possibility. Will get it out into the sunshine after the sun comes up so that I can have a good look underneath. Would be great if a slipped belt is all it was. As always, thanks for your assistance. My property is loaded with trees, and, this time of year, my yard is loaded with fallen leaves. I completed the front using a Toro 22" walk-behind which is "self-propelled", but emptying the bag into a wagon, pulling the wagon to the back of my property to dump it over and over is wearing on this 74 year-old body. Worked at it all day yesterday. Finished the front, but that's the smallest part of my lawn. Riding is a piece of cake. Thanks again for your help.
Caruso