I cleaned the innards with some carb cleaner, oiled some things, put it all back together, recharged the battery and now it runs. I have no idea what I did, but thanks for all the tips.
It meaures 12.7v with the key off then drops down to ~9.8v when I try to start it. But it still spins better than it did before - probably because I have the carb. and valve cover off. I guess I need to buy a new battery
I have an old 28D7070123-01 11.5HP engine on an MTD riding mower. When I try starting it, the flywheel only turns part way then stops. I also hear a wheezing sound that sounds like pressure being released. I tried starting once with the sparkplug removed and the the flywheel rotated normally...