I'm waiting on a parts kit for a Walbro LMT carb. The riding mower it was installed on was fine until it started suddenly racing. It would not throttle down even with the throttle on "turtle". I was puzzled--checked the governor linkage for damage then consulted Youtube University. Found a video and then I just knew what it was.

The throttle valve (butterfly) had disconnected from the throttle shaft. That means wide open throttle without governor control. The two screws holding it on had backed off and the valve ended up lodged in the intake. Wild! I guess one or both screws had vibrated loose and are either still in the cylinder, perhaps embedded in the cylinder head or the top of the piston, or have exited and are in the muffler.
Will remove the cylinder head to look for the screws, inspect, de-carbonize and replace the head gasket while I am in there. Briggs and Stratton 313707 0162-E1 010508ZE. 16.5 HP OHV.