Briggs & Stratton 18 hp twin
Model 422777
Type 128101

Engine on riding mower was leaking oil badly. I replaced the PTO side seal, oil pump seal, sump gasket, and two head gaskets. Engine started on first try, but will not throttle down--runs wide open. What could I have done to make this happen and how do I fix it?

 

Look at the throttle linkage and see if it's moving properly.
100% sure you installed the governor spring in the right hole and did not bend or bind it?

 

Also, when you had the sump cover off, if the governor gear assembly shifts out of place while you put the cover on, it will not work. You might have to pull it back off if you can find no problem with the throttle or linkage.