Have you taken a good look at the exhaust/muffler?

Yeah, that was one of the first things I checked.

I pulled the plastic stops off of the carb adjustment screws.
By backing them out about 1 1/2 turns (WAY more than the stops
allowed), I was able to get it to run. It still won't go full
throttle, and I had to keep massaging the throttle to keep it from
dying, but I was able to move some dirt around.

I guess I'll take the carb apart over the Winter.