Yeah, I had that happen last year (nest in the engine). It was disgusting! They royally messed it up in there. I started it, not realizing. Mouse guts and other parts are horrible to try to clean out . . .
I purchased my first rider last year (new) and was told by a friend to make sure to stuff steel wool in the exhaust pipe at the end of the season so that mice don't get in there in the winter. I did that, and everything turned out fine.
Good advice!
