Just so we can be certain of the exact beastie we're dealing with here, there are a set of 3 numbers which are the key to Briggs engine ID. They are the 'Model, Type and Code', and will be in that order. See if you can post them up, if at all possible.
On this engine, they should be stamped in two places - the inner metal cooling cowl, above the spark plug; and on the heat deflector plate above the muffler. Rust may have obscured the latter lot, though.
I'd say that the Briggs Repair Manual you need is P/No 270962, Repair Manual Single Cylinder L-Head, if as I suspect, this is a 12000 model series side valve engine. There are a few free downloads of it around the Web, or it's available for pay [subscription] in the ODK 'Engine Manuals & Parts List' section.
The function of the governor spring is to hold open the throttle; the centrifugal governor acts against it, and backs off the throttle, as engine revs increase. The older print edition Briggs Repair Manuals had a section at the end, that described this concept very well, with diagrams. That all seems to have gone over to their Training Course Textbooks these days, though...
It is surely possible for the governor internals to fail, as it's a part of the oil slinger, that's driven off the camshaft gear. At least the slinger gear is plastic, and on the Quantums the camshaft gear is too, IIRC.
