Here is the Illustrated Parts List:
http://bsintek.basco.com/BriggsDocumentDisplay/Z6ntzHVJ1DajI.pdf

Here is the Operator's Manual:
http://bsintek.basco.com/BriggsDocumentDisplay/mssAIQ-K_ny7tH217FzoPu.pdf

Tank, I dreamed up a possible explanation of what is going on. Please test it against what your engine is doing.

Suppose the needle and seat set is leaking, so when the mower is standing idle, the float bowl slowly over-fills with fuel. This means it will start rich, even if you don't use the choke. Suppose the previous tenant was troubled by this but instead of fixing the problem by replacing the needle and seat, he adjusted the main mixture control toward lean. The outcome would be, the engine would still start rich, and would remain rich for perhaps several minutes while the float level slowly lowered itself until the needle and seat began to operate correctly. The problem, however, would be that he'd adjusted the mixture to be lean, so it would hunt after this several minutes had passed.

There is a simple way to test whether this is what has happened. After running for several minutes, so the engine begins to hunt, turn the main mixture control (Item 4 in the illustration below) anticlockwise when viewed from below (that is, screw it out) a fraction of a turn and see if the hunting is eliminated. Turn it back and forth until you have it running as well as possible, and see if it is then running properly.

[Linked Image]


Of course after it has been standing with the fuel turned on, it will start, and initially run, richer than ever, and this will have to be dealt with by servicing the carburetor.

The full details on overhauling all of the B&S side valve single cylinder engines made after 1984 can be found in B&S Part Number 270962, Single Cylinder L Head Tech Manual. If you can't find a free download anywhere on the internet, that and all of the other Briggs tech manuals can be downloaded from Outdoorking, but an annual subscription is required. Alternatively you can just follow the process in the thread I referred you to previously. You may notice that 168,000 people have already referred to that thread, which may give you some idea how common worn needle-and seat is in very old 28 cubic inch Briggs engines.