After cleaning up the carb on my wife's riding mower it was running fine, better than it has in a couple years anyway, only a little missing at full throttle under no load, idled great. Worked good for several days, then she's mowing along and all of a sudden the RPM goes way up. Pulled the throttle back but the rpms hardly changed. She drove it back to the garage so I could check it out, which I haven't done yet - but there was a clanking sound when she shut it off, think maybe it threw a rod? Haven't torn it open yet.

Anyway, my question is, what usually causes an engine to suddenly increase speed like that? The carb setting was fine, linkages all seem to work freely.

Engine is a Briggs Intek OHV 18 HP on a Craftsman 42" mower.

Update - just now getting around to tearing into this thing - started with the top end cuz it's easier... took the head off and whatta mess! Carbon and varnish all over the piston face and the valve head. This thing was running poor for a while before I cleaned out the carb, apparently bad gas made a big mess. But the big surprise is the little steel screw stuck into the top of the piston! :freak: Where'd that come from? :confused:
http://img177.imageshack.us/img177/2498/pistonhead1js2.jpg
http://img516.imageshack.us/img516/8398/pistonhead2cx2.jpg

All I can think of is maybe the governor came apart and the screw somehow got sucked up through the crankcase vent tube, through the carb, into the chamber? Is that even possible? I can't imagine this thing would have run for hours with that screw banging around in there without destroying something. The cylinder walls look fine.

That screw came out of your carburetor. It's one of the screws that holds the throttle or choke plate on it's shaft.

Take your carburetor off and inspect the throttle and choke and you should find a place for the screw to go.

This could also be the reason your engine raced, if the screw came out of the throttle plate and the plate came of the throttle shaft.

You nailed it! In fact both screws came off the throttle plate, one of them somehow made it all the way out into the muffler, where it will have to remain as I can't get it out. The throttle plate itself lodged in the intake tube, thankfully. I think you're correct that this explains the racing as well.

Assuming I can get replacement screws, I'll reassemble the throttle. Is there some sort of Loctite that I should use to keep this from happening again? I wonder if this happened because that got dissolved by the carb cleaner.

I'll get some cleaner and tackle the carbon on the piston and head, too. Should I do anything about the dings on the piston face?

I see this happen occasionally, but not that often. When I install new screws (yes they are available), after I tighten them in, I open the throttle to expose the back side and flare out the end of the screws (they are pretty soft) with a flat blade screw driver, so it makes it hard for them to get loose and back out.