I had that happen on a John Deere 316 lawn mower.
It still "ran" just not real well. It had eaten a little screw that had come out of the choke butterfly on the carb. Itty bitty thing. It made it past the valve, bent it a tick so it kinda hung open, rode up and down next to the piston and the wall for a while before it was shut down. Blew the dip stick clean out of the tube, and under the hood looked like that tractor trailer.
I could not find the screw for anything....where the hell did it come from. Finally I noticed one missing in the carb. You have no idea how pissed off I was.