Why does my 8 hp Briggs Stratton idle fast and die when throttle is opened?

It sounds like the governor linkage is frozen, jammed, or out of adjustment. Locate the throttle shaft on the carburetor body, start it up, and put the engine under load. As it dies, you should see the governor link open the throttle completely up, to compensate for the lower engine speed. If it does appear to operate correctly, the problem will be carb related, probably a blocked main jet or air bleed, and should be cleaned and rebuilt.