26 hp v twin model 445877 . Replaced sump pan gasket . Remounted motor and engine runs wide open throttle. Any ideas of problem?

Hi,
You have jammed the governor assembly either internally or externally during reassembly. You can check it externally by checking for freedom of movement of the linkage to the left and below the carb (when facing the air filter/carb side of the engine) and not having it lock or snapping to one position. It could be you had it out of position during assembly or could have shifted it when putting it back together.
I attach a IPL (parts breakdown) and page 7 is the one to look at. Item #616 is the governor shaft that is actuated by #219 governor and operates the carb via #227 governor arm. If everything works frely and is not pinned in on location (with the throttle set to idle) then you would loose the pinch bolt/nut on the arm (throttle set to fully speed) rotate the end of the shaft counter-clockwise fully while holding linkage to full speed and tightening.
Peace,
Clarke